fix(ui): make overview modal resizable; add diff content pane

Drop outer Border wrapper in WorktreesOverviewModalView so Avalonia edge
resize handles reach the window frame. Add split pane to WorktreeModalView
with file tree on left and per-file unified diff on right; wire SelectedNode
via SelectedItem TwoWay binding + SelectionChanged fallback; add
GetFileDiffAsync to GitService.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/ClaudeDo.Data/Git/GitService.cs

@@ -106,6 +106,15 @@ public sealed class GitService
         return stdout.Trim();
     }
 
+    public async Task<string> GetFileDiffAsync(string worktreePath, string? baseCommit, string relativePath, CancellationToken ct = default)
+    {
+        string[] args = string.IsNullOrEmpty(baseCommit)
+            ? ["diff", "--", relativePath]
+            : ["diff", $"{baseCommit}..HEAD", "--", relativePath];
+        var (_, stdout, _) = await RunGitAsync(worktreePath, args, ct);
+        return stdout;
+    }
+
     public async Task WorktreeRemoveAsync(string repoDir, string worktreePath, bool force = false, CancellationToken ct = default)
     {
         var args = new List<string> { "worktree", "remove" };