feat(settings): mount full SettingsScreen with all 12 panels (DCC-D1)

- add get_app_data_dir tauri command returning the app data directory path
- mount SettingsScreen with Account, Appearance, Editor, Sync, Notifications,
  Plugins, ApiKeys, Agents, Hotkeys, Privacy, Advanced, Diagnostics panels
- wire HotkeysPanel with current CmdOrCtrl+Shift+K binding and reset handlers
- wire AdvancedPanel onShowLogs to reveal app data dir via Tauri shell/finder
- wire PrivacyPanel dataDir from get_app_data_dir; onExportData opens dir
- wire ApiKeysPanel to /api/api-keys endpoints (mint + revoke)
- wire PluginsPanel to /api/plugins/* (toggle enable/disable, install by url)
- wire SyncPanel to adapter.triggerSync(); show server url and last sync time
- wire DiagnosticsPanel onCopyReport to navigator.clipboard
- add Settings button in AppShell header alongside existing Plugins/Agents buttons
- standalone Agents/Plugins buttons kept for quick access; AgentsPanel also
  present inside SettingsScreen under the Agents tab

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

Author: piwi3910

src-tauri/src/lib.rs

@@ -142,6 +142,17 @@ async fn update_jumplist(
     Ok(())
 }
 
+// --- App data directory ---
+
+#[tauri::command]
+async fn get_app_data_dir(app: tauri::AppHandle) -> Result<String, String> {
+    use tauri::Manager;
+    app.path()
+        .app_data_dir()
+        .map(|p| p.to_string_lossy().into_owned())
+        .map_err(|e| e.to_string())
+}
+
 #[cfg_attr(mobile, tauri::mobile_entry_point)]
 pub fn run() {
     tauri::Builder::default()
@@ -203,6 +214,8 @@ pub fn run() {
             win_search_upsert_note,
             win_search_remove_note,
             update_jumplist,
+            // Phase D: app data dir for settings panels
+            get_app_data_dir,
         ])
         .run(tauri::generate_context!())
         .expect("error while running tauri application");