perf(bundle): lazy-load rarely-visited pages + split vendor chunks (#249)

Cold-start bundle was a single 1.88 MB / 577 KB-gzipped chunk that
parsed every page in the app before first paint, including pages a
typical session never opens.

Two-part fix:

1. Lazy-load 10 rarely-visited pages via React.lazy() — each becomes
   its own chunk that only downloads when the route is hit:
     Settings, Statistics, Help, WhatsNew, DeviceSync, OfflineLibrary,
     LabelAlbums, AdvancedSearch, FolderBrowser, InternetRadio.
   Whole <Routes> tree is wrapped in <Suspense fallback={null}> — no
   visible loading state, the browser cache hits on subsequent visits.

2. Vite manualChunks splits dependencies that change rarely from app
   code: react/react-dom/react-router, the @tauri-apps/* family, and
   i18next. Tauri auto-updater pulls smaller deltas when only app
   code changed (vendor chunks stay byte-identical and cached).

Build output:
  Before: 1.88 MB / 577 KB gz monolithic
  After cold start: ~450 KB gz (index 370 + react 54 + tauri 6 + i18n 20)
                    = 22% smaller initial download
  Plus 11 lazy chunks. Notably WhatsNew's bundled CHANGELOG (215 KB /
  76 KB gz) is now off the cold-start path entirely.

Co-authored-by: Psychotoxical <dev@psysonic.app>
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

vite.config.ts

@@ -27,5 +27,25 @@ export default defineConfig({
     target: process.env.TAURI_ENV_PLATFORM === "windows" ? "chrome105" : "safari13",
     minify: !process.env.TAURI_ENV_DEBUG ? "esbuild" : false,
     sourcemap: !!process.env.TAURI_ENV_DEBUG,
+    rollupOptions: {
+      output: {
+        // Vendor chunks isolate dependencies that change rarely from app code,
+        // so a normal app update doesn't invalidate the cached vendor bundles
+        // (helps especially with the Tauri updater pulling deltas).
+        manualChunks: {
+          react: ["react", "react-dom", "react-router-dom"],
+          tauri: [
+            "@tauri-apps/api",
+            "@tauri-apps/plugin-shell",
+            "@tauri-apps/plugin-dialog",
+            "@tauri-apps/plugin-fs",
+            "@tauri-apps/plugin-process",
+            "@tauri-apps/plugin-store",
+            "@tauri-apps/plugin-updater",
+          ],
+          i18n: ["i18next", "react-i18next"],
+        },
+      },
+    },
   },
 });