feat: thumbnail picker custom per selezione modello/scena

- GET /folder_image/{filename}?w=N: PNG ridotto cache 1h
- Frontend: 2 thumb-picker al posto dei select (thumb + nome + caret)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

pm2d/web/static/index.html

@@ -10,13 +10,21 @@
   <h1>Pattern Matching 2D</h1>
   <div class="toolbar">
     <label class="tb-label">Modello:</label>
-    <select id="sel-model" class="tb-select">
-      <option value="">-- seleziona --</option>
-    </select>
+    <div class="thumb-picker" id="picker-model">
+      <div class="picker-current" tabindex="0">
+        <span class="picker-text">-- seleziona --</span>
+        <span class="caret">▾</span>
+      </div>
+      <div class="picker-list"></div>
+    </div>
     <label class="tb-label">Scena:</label>
-    <select id="sel-scene" class="tb-select">
-      <option value="">-- seleziona --</option>
-    </select>
+    <div class="thumb-picker" id="picker-scene">
+      <div class="picker-current" tabindex="0">
+        <span class="picker-text">-- seleziona --</span>
+        <span class="caret">▾</span>
+      </div>
+      <div class="picker-list"></div>
+    </div>
     <button class="btn btn-go" id="btn-match">▶ MATCH</button>
     <span id="status">Seleziona modello, disegna ROI, seleziona scena</span>
   </div>