fix(display): auto-detect displays for mirror/extend via displayplacer list

The handler was a stub that required an explicit configStr — the Svelte
caller never provides one, so mirror/extend silently failed every launch.

Now auto-detects connected displays:
- mirror: parses displayplacer list output, adds mirror:<primary_id> to
  secondary display(s); removes any existing mirror key first.
- extend: re-applies current layout if already extended; otherwise strips
  mirror keys and computes side-by-side origins from each display's width.
- configStr still takes priority when provided (manual per-device override).

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

src/main/system_handlers.ts

@@ -319,6 +319,9 @@ export function registerSystemHandlers() {
   });
 
   // 6. Set Display Layout (Displayplacer)
+  // Auto-detects connected displays via `displayplacer list` when no explicit configStr is given.
+  // mirror: secondary display(s) mirror the primary.
+  // extend: displays shown side-by-side; un-mirrors if currently mirrored.
   ipcMain.handle('native:set-display-layout', async (event, { mode, configStr }) => {
     if (os.platform() !== 'darwin') return { success: false, error: 'Display control only supported on macOS' };
 
@@ -326,27 +329,68 @@ export function registerSystemHandlers() {
       ? path.join(process.resourcesPath, 'bin', 'displayplacer')
       : path.join(__dirname, '../../resources/bin/displayplacer');
 
-    let cmd = '';
+    // Explicit config string always takes priority — allows manual override per device.
+    if (configStr) {
+      return await runExec(`"${binPath}" ${configStr}`);
+    }
+
+    // Auto-detect: `displayplacer list` emits a ready-to-run command line at the bottom.
+    // We parse the quoted display strings from that line and modify them for the requested mode.
+    const list_result = await runExec(`"${binPath}" list`);
+    if (!list_result.success || !list_result.stdout) {
+      return { success: false, error: `displayplacer list failed: ${list_result.error ?? 'no output'}` };
+    }
+
+    // The command line looks like: displayplacer "id:xxx res:... origin:(0,0) ..." "id:yyy ..."
+    const cmd_line = list_result.stdout.split('\n').find(l => l.trim().startsWith('displayplacer "'));
+    if (!cmd_line) {
+      return { success: false, error: 'Only one display connected or displayplacer list output unrecognised' };
+    }
+
+    const display_strings = [...cmd_line.matchAll(/"([^"]+)"/g)].map(m => m[1]);
+    if (display_strings.length < 2) {
+      return { success: false, error: 'Only one display found; cannot change layout' };
+    }
 
     if (mode === 'mirror') {
-        // This usually requires querying current IDs, which is complex.
-        // If configStr is provided (output of 'displayplacer list'), use it.
-        if (configStr) {
-            cmd = `"${binPath}" ${configStr}`;
-        } else {
-             return { success: false, error: 'Config string required for now' };
-        }
-    } else if (mode === 'extend') {
-        if (configStr) {
-            cmd = `"${binPath}" ${configStr}`;
-        }
+      const primary_id_match = display_strings[0].match(/\bid:([^\s]+)/);
+      if (!primary_id_match) {
+        return { success: false, error: 'Could not parse primary display ID from displayplacer output' };
+      }
+      const primary_id = primary_id_match[1];
+
+      // Primary display unchanged; secondary display(s) get mirror:<primary_id>.
+      const mirror_args = display_strings.map((s, i) => {
+        if (i === 0) return `"${s}"`;
+        const without_existing_mirror = s.replace(/\s*mirror:\S+/g, '').trim();
+        return `"${without_existing_mirror} mirror:${primary_id}"`;
+      }).join(' ');
+
+      return await runExec(`"${binPath}" ${mirror_args}`);
     }
 
-    if (cmd) {
-        return await runExec(cmd);
+    if (mode === 'extend') {
+      const any_mirrored = display_strings.some(s => /\bmirror:\S+/.test(s));
+      if (!any_mirrored) {
+        // Already extended — re-apply current layout to ensure it is active.
+        return await runExec(`"${binPath}" ${display_strings.map(s => `"${s}"`).join(' ')}`);
+      }
+
+      // Remove mirror keys and compute side-by-side origins from each display's resolution.
+      let x_offset = 0;
+      const extend_args = display_strings.map((s) => {
+        const without_mirror = s.replace(/\s*mirror:\S+/g, '').trim();
+        const res_match = without_mirror.match(/\bres:(\d+)x\d+/);
+        const width = res_match ? parseInt(res_match[1]) : 1920;
+        const updated = without_mirror.replace(/\borigin:\([^)]+\)/, `origin:(${x_offset},0)`);
+        x_offset += width;
+        return `"${updated}"`;
+      }).join(' ');
+
+      return await runExec(`"${binPath}" ${extend_args}`);
     }
 
-    return { success: false, error: 'Invalid mode or missing config' };
+    return { success: false, error: `Unsupported display mode: ${mode}` };
   });
 
   // 7. Update App