feat(launcher): add display mode toggle; fix silent display layout failures

- Add Extend/Mirror toggle to Native OS config section (always visible,
  no Technical Mode required). Default: Extend. State updates on success.
- Replace .catch(() => {}) swallowing with console.warn logging on both
  set_display_layout call sites so failures appear in the Electron console
- Remove old edit-mode-only Extend button (replaced by new toggle)
- Update PROJECT doc: displayplacer install note, binary lookup order, GitHub link
- Clean up TODO__Agents.md: resolve stale items, add new low-priority Electron items

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

src/routes/events/[event_id]/(launcher)/launcher_file_cont.svelte

@@ -219,9 +219,8 @@ async function handle_open_file() {
         // URL presentations may still want to set display mode (e.g. Google Slides → extend)
         if (profile.display_mode !== 'none') {
             open_file_status_message = `Setting display (${profile.display_mode})...`;
-            await native.set_display_layout({ mode: profile.display_mode }).catch(() => {
-                /* No external display or displayplacer unconfigured — continue */
-            });
+            const disp_res = await native.set_display_layout({ mode: profile.display_mode }).catch((e: any) => ({ success: false, error: String(e) }));
+            if (!disp_res?.success) console.warn('[Launcher] set_display_layout:', disp_res?.error ?? 'no external display');
         }
 
         open_file_status_message = `Opening ${event_file_obj.title || 'URL'}...`;
@@ -357,9 +356,8 @@ async function handle_open_file() {
         // --- Step 3: Set display layout (skip silently on failure / no external display) ---
         if (profile.display_mode !== 'none') {
             open_file_status_message = `Setting display (${profile.display_mode})...`;
-            await native.set_display_layout({ mode: profile.display_mode }).catch(() => {
-                /* No external display or displayplacer unavailable — continue */
-            });
+            const disp_res = await native.set_display_layout({ mode: profile.display_mode }).catch((e: any) => ({ success: false, error: String(e) }));
+            if (!disp_res?.success) console.warn('[Launcher] set_display_layout:', disp_res?.error ?? 'no external display');
         }
 
         // --- Step 4: Open the file ---