fix image stale-render on in-place overwrite (T-312)

Image cards, thumbnails, the lightbox, and `clide image show` rendered
via Image.file, whose FileImage keys Flutter's imageCache by (path,
scale) only — so overwriting a file at the same path handed back the
previously decoded frame (hit live re-exporting a wireframe PNG). Add
ClideFileImage, a FileImage that folds mtime + size into ==/hashCode so an
in-place change is a fresh cache key (miss → re-decode), and route the
five Image.file sites through it.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/widgets/src/clide_file_image.dart

@@ -0,0 +1,39 @@
+import 'dart:io';
+
+import 'package:flutter/widgets.dart';
+
+/// A [FileImage] whose cache key also folds in the file's modification time and
+/// size — so re-showing a path whose bytes changed *in place* re-decodes
+/// instead of returning Flutter's stale cached frame (T-312).
+///
+/// Plain `Image.file` / `FileImage` key the global `imageCache` by `(path,
+/// scale)` only, so overwriting a file at the same path is invisible: the cache
+/// hands back the previously decoded image. Hit live when re-exporting a
+/// wireframe PNG in place kept showing the prior render. Folding mtime + size
+/// into `==`/`hashCode` makes an overwrite a fresh key → a cache miss → a
+/// re-read of the current bytes; an unchanged file still hits the cache.
+class ClideFileImage extends FileImage {
+  ClideFileImage(String path, {double scale = 1.0})
+      : _stamp = _stampOf(path),
+        super(File(path), scale: scale);
+
+  /// mtime ⊕ size — changes on any in-place overwrite (a write bumps mtime; a
+  /// different length bumps size even within one clock tick). 0 if the file
+  /// can't be stat'd, which falls back to plain path+scale keying.
+  final int _stamp;
+
+  static int _stampOf(String path) {
+    try {
+      final s = File(path).statSync();
+      return s.modified.millisecondsSinceEpoch ^ s.size;
+    } catch (_) {
+      return 0;
+    }
+  }
+
+  @override
+  bool operator ==(Object other) => other is ClideFileImage && other.file.path == file.path && other.scale == scale && other._stamp == _stamp;
+
+  @override
+  int get hashCode => Object.hash(file.path, scale, _stamp);
+}