fix: wire up Default Tool View save in General settings

The <select> for Default Tool View was an uncontrolled dead control with
no value binding, no onChange handler, and no save mechanism. This wires
it up end-to-end:

- Add defaultToolView to the Zustand settings store
- Load the persisted value from the settings API on mount
- Bind the <select> with value/onChange
- Add Save Settings button mirroring SystemSection's pattern
- Redirect home page to /fullscreen when defaultToolView is "fullscreen"

Closes #75

apps/web/src/pages/home-page.tsx

@@ -24,7 +24,7 @@ export function HomePage() {
     currentEntry,
   } = useFileStore();
   const navigate = useNavigate();
-  const { fetch: fetchSettings } = useSettingsStore();
+  const { fetch: fetchSettings, defaultToolView, loaded: settingsLoaded } = useSettingsStore();
   const { fetch: fetchFeatures, isToolInstalled } = useFeaturesStore();
 
   useEffect(() => {
@@ -32,6 +32,12 @@ export function HomePage() {
     fetchFeatures();
   }, [fetchSettings, fetchFeatures]);
 
+  useEffect(() => {
+    if (settingsLoaded && defaultToolView === "fullscreen" && files.length === 0) {
+      navigate("/fullscreen", { replace: true });
+    }
+  }, [settingsLoaded, defaultToolView, files.length, navigate]);
+
   const handleFiles = useCallback(
     (newFiles: File[]) => {
       reset();