feat: migrate GitHub stars fetch to TanStack Query

Replace the manual useEffect + fetch in NavbarGitHubStars with
useQuery from @tanstack/react-query. This gives the component
caching, request deduplication, automatic retries, and devtools
visibility for free.

The useLatestRelease hook referenced in the issue has already been
removed from hero-section.tsx, so only the stars badge needed
migration.

Closes #1198

surfsense_web/lib/query-client/cache-keys.ts

@@ -91,6 +91,10 @@ export const cacheKeys = {
 	publicChat: {
 		byToken: (shareToken: string) => ["public-chat", shareToken] as const,
 	},
+	github: {
+		repoStars: (username: string, repo: string) =>
+			["github", "repo-stars", username, repo] as const,
+	},
 	publicChatSnapshots: {
 		all: ["public-chat-snapshots"] as const,
 		bySearchSpace: (searchSpaceId: number) =>