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/stores/settings-store.ts

@@ -4,6 +4,7 @@ import { apiGet } from "@/lib/api";
 interface SettingsState {
   disabledTools: string[];
   experimentalEnabled: boolean;
+  defaultToolView: "sidebar" | "fullscreen";
   loaded: boolean;
   fetch: () => Promise<void>;
 }
@@ -11,6 +12,7 @@ interface SettingsState {
 export const useSettingsStore = create<SettingsState>((set, get) => ({
   disabledTools: [],
   experimentalEnabled: false,
+  defaultToolView: "sidebar",
   loaded: false,
 
   fetch: async () => {
@@ -23,10 +25,10 @@ export const useSettingsStore = create<SettingsState>((set, get) => ({
       set({
         disabledTools: data.settings.disabledTools ? JSON.parse(data.settings.disabledTools) : [],
         experimentalEnabled: data.settings.enableExperimentalTools === "true",
+        defaultToolView: data.settings.defaultToolView === "fullscreen" ? "fullscreen" : "sidebar",
         loaded: true,
       });
     } catch {
-      // Settings fetch failed - default to no disabled tools
       set({ loaded: true });
     }
   },