feat(frontend): add API client, layout, and routing

- Create Axios-based API client with sources, jobs, and dashboard endpoints
- Add responsive Layout component with navigation sidebar
- Set up React Router with Dashboard, Backups, and Settings routes
- Configure TanStack Query provider with default options
- Use lucide-react for navigation icons

frontend/node_modules/internal-slot/index.d.ts

@@ -0,0 +1,12 @@
+declare namespace SLOT {
+	type InternalSlot = string; // `[[${string}]]`; // TODO: restrict this to require the brackets
+}
+
+declare const SLOT: {
+	assert(O: object, slot: SLOT.InternalSlot): void;
+	get(O: object, slot: SLOT.InternalSlot): unknown;
+	set(O: object, slot: SLOT.InternalSlot, value?: unknown): void;
+	has(O: object, slot: SLOT.InternalSlot): boolean;
+}
+
+export = SLOT;