refactor: extract app bootstrap to $lib/app/bootstrap.ts

Create bootstrap module with:
- loadSavedTheme() - theme initialization
- restoreLastfmSession() - Last.fm session restore
- setupMouseNavigation() - mouse back/forward handlers
- bootstrapApp() - combined init with cleanup

+page.svelte: 1,229 → 1,194 lines (-35 lines)

src/lib/app/bootstrap.ts

@@ -0,0 +1,102 @@
+/**
+ * App Bootstrap
+ *
+ * Handles application startup tasks that don't depend on component state.
+ * This includes theme initialization, Last.fm session restore, etc.
+ */
+
+import { invoke } from '@tauri-apps/api/core';
+import { goBack, goForward } from '$lib/stores/navigationStore';
+
+// ============ Theme Management ============
+
+/**
+ * Load and apply saved theme from localStorage
+ */
+export function loadSavedTheme(): void {
+  const savedTheme = localStorage.getItem('qbz-theme');
+  if (savedTheme) {
+    document.documentElement.setAttribute('data-theme', savedTheme);
+  }
+}
+
+// ============ Last.fm Session ============
+
+/**
+ * Restore Last.fm session from localStorage
+ */
+export async function restoreLastfmSession(): Promise<void> {
+  try {
+    const savedApiKey = localStorage.getItem('qbz-lastfm-api-key');
+    const savedApiSecret = localStorage.getItem('qbz-lastfm-api-secret');
+    const savedSessionKey = localStorage.getItem('qbz-lastfm-session-key');
+
+    // Restore credentials if user-provided
+    if (savedApiKey && savedApiSecret) {
+      await invoke('lastfm_set_credentials', {
+        apiKey: savedApiKey,
+        apiSecret: savedApiSecret
+      });
+    }
+
+    // Restore session if available
+    if (savedSessionKey) {
+      await invoke('lastfm_set_session', { sessionKey: savedSessionKey });
+      console.log('Last.fm session restored on startup');
+    }
+  } catch (err) {
+    console.error('Failed to restore Last.fm session:', err);
+  }
+}
+
+// ============ Mouse Navigation ============
+
+/**
+ * Handle mouse back/forward buttons
+ */
+function handleMouseNavigation(event: MouseEvent): void {
+  if (event.button === 3) {
+    event.preventDefault();
+    goBack();
+  } else if (event.button === 4) {
+    event.preventDefault();
+    goForward();
+  }
+}
+
+/**
+ * Setup mouse navigation event listener
+ * @returns Cleanup function to remove listener
+ */
+export function setupMouseNavigation(): () => void {
+  window.addEventListener('mouseup', handleMouseNavigation);
+  return () => window.removeEventListener('mouseup', handleMouseNavigation);
+}
+
+// ============ Combined Bootstrap ============
+
+export interface BootstrapResult {
+  cleanup: () => void;
+}
+
+/**
+ * Bootstrap the application
+ * Call this in onMount to initialize app-level features
+ * @returns Object with cleanup function for onDestroy
+ */
+export function bootstrapApp(): BootstrapResult {
+  // Load theme
+  loadSavedTheme();
+
+  // Setup mouse navigation
+  const cleanupMouse = setupMouseNavigation();
+
+  // Restore Last.fm session (async, fire-and-forget)
+  restoreLastfmSession();
+
+  return {
+    cleanup: () => {
+      cleanupMouse();
+    }
+  };
+}

src/routes/+page.svelte

@@ -161,6 +161,9 @@
     shareSonglinkTrack
   } from '$lib/services/trackActions';
 
+  // App bootstrap
+  import { bootstrapApp } from '$lib/app/bootstrap';
+
   // Components
   import Sidebar from '$lib/components/Sidebar.svelte';
   import NowPlayingBar from '$lib/components/NowPlayingBar.svelte';
@@ -794,24 +797,11 @@
   let downloadStateVersion = $state(0);
 
   onMount(() => {
-    // Load saved theme
-    const savedTheme = localStorage.getItem('qbz-theme');
-    if (savedTheme) {
-      document.documentElement.setAttribute('data-theme', savedTheme);
-    }
-
-    const handleMouseNavigation = (event: MouseEvent) => {
-      if (event.button === 3) {
-        event.preventDefault();
-        navGoBack();
-      } else if (event.button === 4) {
-        event.preventDefault();
-        navGoForward();
-      }
-    };
+    // Bootstrap app (theme, mouse nav, Last.fm restore)
+    const { cleanup: cleanupBootstrap } = bootstrapApp();
 
+    // Keyboard navigation
     document.addEventListener('keydown', handleKeydown);
-    window.addEventListener('mouseup', handleMouseNavigation);
 
     // Initialize download states
     initDownloadStates();
@@ -887,34 +877,9 @@
       }
     });
 
-    // Restore Last.fm session on app startup
-    (async () => {
-      try {
-        const savedApiKey = localStorage.getItem('qbz-lastfm-api-key');
-        const savedApiSecret = localStorage.getItem('qbz-lastfm-api-secret');
-        const savedSessionKey = localStorage.getItem('qbz-lastfm-session-key');
-
-        // Restore credentials if user-provided
-        if (savedApiKey && savedApiSecret) {
-          await invoke('lastfm_set_credentials', {
-            apiKey: savedApiKey,
-            apiSecret: savedApiSecret
-          });
-        }
-
-        // Restore session if available
-        if (savedSessionKey) {
-          await invoke('lastfm_set_session', { sessionKey: savedSessionKey });
-          console.log('Last.fm session restored on startup');
-        }
-      } catch (err) {
-        console.error('Failed to restore Last.fm session:', err);
-      }
-    })();
-
     return () => {
+      cleanupBootstrap();
       document.removeEventListener('keydown', handleKeydown);
-      window.removeEventListener('mouseup', handleMouseNavigation);
       stopDownloadEventListeners();
       unsubscribeDownloads();
       unsubscribeToast();