Add executable picker in game edit UI

- Added 'executables' (full list) and 'executable' (selected) fields to Game
- Upload now stores ALL discoverable .exe/.com/.bat files in game metadata
- Added setExecutable() to GameService that patches the .jsdos bundle
  (updates dosbox.conf + jsdos.json inside the ZIP) when changing executable
- GameResource PATCH handler delegates executable changes to bundle patching
- GameDetail edit mode shows a radio-button picker of all executables
- Heuristics remain as first-guess default, user can always override
- No re-upload needed to fix wrong executable selection

frontend/src/pages/GameDetail.svelte

@@ -17,6 +17,7 @@
   let editPublisher = $state("");
   let editDescription = $state("");
   let editPlatform = $state("");
+  let editExecutable = $state("");
 
   // IGDB scrape state
   let scraping = $state(false);
@@ -65,6 +66,7 @@
     editPublisher = game.publisher || "";
     editDescription = game.description || "";
     editPlatform = game.platform || "";
+    editExecutable = game.executable || "";
     editing = true;
   }
 
@@ -79,6 +81,7 @@
         publisher: editPublisher || undefined,
         description: editDescription || undefined,
         platform: editPlatform || undefined,
+        executable: editExecutable || undefined,
       });
       editing = false;
     } catch (e) {
@@ -207,6 +210,25 @@
               <option value="windows">Windows</option>
             </select>
           </div>
+          {#if game.executables && game.executables.length > 0}
+            <div class="edit-exec-section">
+              <span class="edit-exec-label">Main executable:</span>
+              <div class="edit-exec-list">
+                {#each game.executables as exe}
+                  <label class="edit-exec-option" class:selected={editExecutable === exe}>
+                    <input
+                      type="radio"
+                      name="executable"
+                      value={exe}
+                      checked={editExecutable === exe}
+                      onchange={() => editExecutable = exe}
+                    />
+                    <span class="edit-exec-path">{exe}</span>
+                  </label>
+                {/each}
+              </div>
+            </div>
+          {/if}
           <div class="edit-actions">
             <button class="btn btn-primary" onclick={saveEdit} disabled={saving}>
               {saving ? "Saving..." : "Save"}
@@ -601,6 +623,62 @@
     color: #66ff66;
   }
 
+  /* ─── Executable Picker ────────────────────── */
+  .edit-exec-section {
+    margin-top: 12px;
+    padding-top: 12px;
+    border-top: 1px solid var(--border);
+  }
+  .edit-exec-label {
+    display: block;
+    font-size: 0.8rem;
+    color: var(--text-dim);
+    font-weight: 600;
+    text-transform: uppercase;
+    letter-spacing: 0.05em;
+    margin-bottom: 8px;
+  }
+  .edit-exec-list {
+    display: flex;
+    flex-direction: column;
+    gap: 4px;
+    max-height: 180px;
+    overflow-y: auto;
+  }
+  .edit-exec-option {
+    display: flex;
+    align-items: center;
+    gap: 8px;
+    padding: 6px 8px;
+    border-radius: var(--radius-sm);
+    cursor: pointer;
+    font-size: 0.85rem;
+    font-family: var(--font-mono);
+    background: var(--surface);
+    border: 1px solid var(--border);
+    transition: border-color 0.15s, background 0.15s;
+  }
+  .edit-exec-option:hover {
+    border-color: var(--phosphor-dim);
+    background: var(--surface-hover);
+  }
+  .edit-exec-option.selected {
+    border-color: var(--phosphor);
+    background: var(--phosphor-burn);
+  }
+  .edit-exec-option input[type="radio"] {
+    accent-color: var(--phosphor);
+    margin: 0;
+    flex-shrink: 0;
+  }
+  .edit-exec-path {
+    word-break: break-all;
+    color: var(--text);
+  }
+  .edit-exec-option.selected .edit-exec-path {
+    color: var(--phosphor-glow);
+  }
+
   /* ─── Responsive ────────────────────────────────── */
   @media (max-width: 640px) {
     .detail { padding: 16px 0; }