feat(FN-1277): hydrate dashboard theme settings from backend

- Hydrate useTheme from global settings while seeding initial state from localStorage cache to prevent flash
- Add write-through theme persistence so setter updates local cache immediately and asynchronously saves via updateGlobalSettings
- Clarify Settings modal save behavior for intentional idempotent global theme writes
- Expand useTheme and App tests to cover backend hydration, fallback handling, and new global settings fetch expectations

packages/dashboard/app/components/SettingsModal.tsx

@@ -500,7 +500,10 @@ export function SettingsModal({
         }
       }
 
-      // Save both scopes in parallel if they have changes
+      // Save both scopes in parallel if they have changes.
+      // Note: themeMode/colorTheme may also be write-through via useTheme callbacks
+      // in the Appearance section; duplicate global writes are intentional/idempotent,
+      // while this save path persists the full settings form in one action.
       await Promise.all([
         Object.keys(globalPatch).length > 0 ? updateGlobalSettings(globalPatch) : Promise.resolve(),
         Object.keys(projectPatch).length > 0 ? updateSettings(projectPatch, projectId) : Promise.resolve(),

packages/dashboard/app/components/__tests__/App.test.tsx

@@ -538,7 +538,7 @@ describe("App auto-open Settings on unauthenticated", () => {
   });
 
   it("auto-opens Settings to Authentication tab when all providers are unauthenticated but onboarding IS complete", async () => {
-    (fetchGlobalSettings as ReturnType<typeof vi.fn>).mockResolvedValueOnce({
+    (fetchGlobalSettings as ReturnType<typeof vi.fn>).mockResolvedValue({
       modelOnboardingComplete: true,
     });
 
@@ -567,7 +567,7 @@ describe("App auto-open Settings on unauthenticated", () => {
         { id: "github", name: "GitHub", authenticated: false },
       ],
     });
-    (fetchGlobalSettings as ReturnType<typeof vi.fn>).mockResolvedValueOnce({
+    (fetchGlobalSettings as ReturnType<typeof vi.fn>).mockResolvedValue({
       defaultProvider: "anthropic",
       defaultModelId: "claude-sonnet-4-5",
     });
@@ -591,7 +591,7 @@ describe("App auto-open Settings on unauthenticated", () => {
       ],
     });
     // No defaultProvider or defaultModelId → setup incomplete
-    (fetchGlobalSettings as ReturnType<typeof vi.fn>).mockResolvedValueOnce({
+    (fetchGlobalSettings as ReturnType<typeof vi.fn>).mockResolvedValue({
       modelOnboardingComplete: false,
     });