feat(postit): signature capture page (dev entry, file persistence)

Builds on f9cfd560 (SignaturePadControl + SignaturePadData) with a
full Avalonia page that captures signatures, renders them as Polylines,
and persists the wire-format payload to ~/.local/share/PostIt/signatures
as JSON v1.

Scope
- New SignaturePage (axaml + code-behind) hosts the render-agnostic
  control: a fixed-size Border is the hit-test surface, an overlaid
  Canvas is rebuilt on every RedrawRequested from the Strokes buffer.
- SignaturePageViewModel wraps the control: exposes StrokeCount /
  PointCount / StatusMessage, Clear and CaptureAsync commands, and
  Attach/Detach for view-lifetime ownership.
- CaptureAsync writes a JSON envelope { format, coordinateMax,
  capturedAtUtc, strokes, strokeCount } to
  LocalApplicationData/PostIt/signatures/signature-yyyyMMdd-HHmmssfff.json.
  This is a stop-gap; the production transport will be
  POST /api/signature/{devisId} on Yavsc.Api (commit 3+).
- Entry point is a [DEV] button on MainPage that pushes the page
  onto the NavigationPage. The production trigger is a SignalR push
  from Yavsc.Org ("devis received, sign here") landing on a hub
  handler — the button and its Click handler are explicitly marked
  dev-only and tracked for removal in the same commit that wires
  the SignalR handler.

Plumbing
- App.axaml.cs: SignaturePage and SignaturePageViewModel registered
  as Transient in the DI container.
- ViewLocator: routes SignaturePageViewModel to SignaturePage.
- SignaturePadData: adds PointCount (sum of pairs across strokes),
  used by the VM status bar and the test surface.

Tests (57/57 green, 9 new in this commit)
- SignaturePageViewModelTests: constructors and dimension validation,
  Attach/Detach idempotence, StrokeCompleted and Clear propagate to
  the VM, CaptureAsync on empty buffer is a no-op, CaptureAsync on a
  non-empty buffer writes a v1 envelope with the expected
  structure (parsed back via JsonDocument, not text matching), and
  creates the destination directory if missing.
- All previously-green tests (48) remain green.

Out of scope
- POST /api/signature endpoint on Yavsc.Api (commit 3).
- SignalR handler that opens the page on a "devis received" push.
- Rasterization: this commit only proves capture and persistence;
  the visible ink is a Polyline reconstruction, not a PNG, by
  design (per the wire-format decision in commit 1).

Note on SignaturePadData
- The PointCount property was added after f9cfd560 landed. It is
  folded into this commit rather than amending f9cfd560 to keep
  the existing history readable; the change is mechanical and
  tested by the new SignaturePageViewModelTests.

src/PostIt/PostIt/ViewModels/SignaturePageViewModel.cs

@@ -0,0 +1,185 @@
+using System;
+using System.IO;
+using System.Text;
+using System.Text.Json;
+using System.Threading.Tasks;
+using CommunityToolkit.Mvvm.ComponentModel;
+using CommunityToolkit.Mvvm.Input;
+using PostIt.Controls;
+using PostIt.Models;
+
+namespace PostIt.ViewModels;
+
+/// <summary>
+/// Backing state for <see cref="PostIt.Views.SignaturePage"/>.
+///
+/// The page exists to produce a <see cref="SignaturePadData"/>
+/// (length-prefixed normalised int[]) from a human signature drawn
+/// with the mouse (Desktop) or finger (touch / Android). The page
+/// is a recipient of an external trigger — a SignalR push from
+/// Yavsc.Org telling PostIt "a devis has been sent, sign here" —
+/// so it intentionally has no first-class entry point in
+/// <see cref="MainPage"/>. The only "open" affordance today is a
+/// dev-only shortcut on the blog editor, marked for removal once
+/// the SignalR handler lands.
+///
+/// Output path is the platform-friendly per-user data directory
+/// (XDG_DATA_HOME / AppData / NSDocumentDirectory on iOS). Files
+/// are JSON, one per capture, named
+/// <c>signature-{yyyyMMdd-HHmmssfff}.json</c>. This is a stop-gap
+/// until the Yavsc.Org endpoint exists; the contract there will
+/// be <c>POST /api/signature/{devisId}</c> with this same payload.
+/// </summary>
+public partial class SignaturePageViewModel : ViewModelBase
+{
+    /// <summary>
+    /// Default capture surface, in DIPs. 3:1 ratio matches a
+    /// signature line at the bottom of an A4 contract.
+    /// </summary>
+    public const double DefaultWidth = 600;
+    public const double DefaultHeight = 200;
+
+    [ObservableProperty]
+    public partial string StatusMessage { get; set; } = "Prêt.";
+
+    [ObservableProperty]
+    public partial int StrokeCount { get; set; }
+
+    [ObservableProperty]
+    public partial int PointCount { get; set; }
+
+    [ObservableProperty]
+    public partial string? LastCapturedPath { get; set; }
+
+    public double Width { get; }
+    public double Height { get; }
+
+    private SignaturePadControl? _control;
+
+    public override bool CanNavigateNext
+    {
+        get => false;
+        protected set { _ = value; }
+    }
+
+    public override bool CanNavigatePrevious
+    {
+        get => true;
+        protected set { _ = value; }
+    }
+
+    public SignaturePageViewModel()
+        : this(DefaultWidth, DefaultHeight)
+    {
+    }
+
+    public SignaturePageViewModel(double width, double height)
+    {
+        if (width <= 0) throw new ArgumentOutOfRangeException(nameof(width));
+        if (height <= 0) throw new ArgumentOutOfRangeException(nameof(height));
+        Width = width;
+        Height = height;
+    }
+
+    /// <summary>
+    /// Bind a freshly-constructed (or re-templated) control to this
+    /// VM. Called from the view's code-behind once the control has
+    /// been added to the visual tree and its template applied (so
+    /// <see cref="SignaturePadControl.CaptureArea"/> is wired).
+    /// </summary>
+    public void Attach(SignaturePadControl control)
+    {
+        if (control is null) throw new ArgumentNullException(nameof(control));
+        Detach();
+        _control = control;
+        _control.RedrawRequested += OnRedraw;
+        _control.StrokeCompleted += OnStrokeCompleted;
+        RefreshCounts();
+    }
+
+    public void Detach()
+    {
+        if (_control is null) return;
+        _control.RedrawRequested -= OnRedraw;
+        _control.StrokeCompleted -= OnStrokeCompleted;
+        _control = null;
+    }
+
+    private void OnStrokeCompleted(object? sender, SignaturePadData data)
+    {
+        StatusMessage = $"Trait terminé. {data.StrokeCount} trait(s).";
+        RefreshCounts();
+    }
+
+    private void OnRedraw(object? sender, EventArgs e) => RefreshCounts();
+
+    private void RefreshCounts()
+    {
+        if (_control is null) return;
+        var snap = _control.Snapshot();
+        StrokeCount = snap.StrokeCount;
+        PointCount = snap.PointCount;
+    }
+
+    [RelayCommand]
+    public void Clear()
+    {
+        _control?.Clear();
+        StatusMessage = "Effacé.";
+        RefreshCounts();
+    }
+
+    [RelayCommand]
+    public async Task CaptureAsync()
+    {
+        if (_control is null)
+        {
+            StatusMessage = "Contrôle non attaché.";
+            return;
+        }
+
+        var data = _control.Snapshot();
+        if (data.IsEmpty)
+        {
+            StatusMessage = "Rien à capturer.";
+            return;
+        }
+
+        try
+        {
+            var path = WriteCapture(data);
+            LastCapturedPath = path;
+            StatusMessage = $"Capture enregistrée: {path}";
+        }
+        catch (Exception ex)
+        {
+            StatusMessage = $"Erreur: {ex.Message}";
+        }
+        await Task.CompletedTask;
+    }
+
+    private static string WriteCapture(SignaturePadData data)
+    {
+        var dir = Path.Combine(
+            Environment.GetFolderPath(Environment.SpecialFolder.LocalApplicationData),
+            "PostIt", "signatures");
+        Directory.CreateDirectory(dir);
+
+        var fileName = $"signature-{DateTime.UtcNow:yyyyMMdd-HHmmssfff}.json";
+        var path = Path.Combine(dir, fileName);
+
+        var payload = new
+        {
+            format = "yavsc.signature/v1",
+            coordinateMax = SignaturePadData.CoordinateMax,
+            capturedAtUtc = DateTime.UtcNow,
+            strokes = data.Strokes,
+            strokeCount = data.StrokeCount,
+        };
+        File.WriteAllText(
+            path,
+            JsonSerializer.Serialize(payload, new JsonSerializerOptions { WriteIndented = true }),
+            Encoding.UTF8);
+        return path;
+    }
+}