fix: proactively refresh Supabase JWT before API calls

Adds token expiry checking and automatic refresh to prevent intermittent
401 errors when the cached session token expires between interactions.

- Check token expiry (60s buffer) before each API call
- Add 401 interceptor that retries once with refreshed token
- Explicitly enable autoRefreshToken in Supabase client

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/lib/supabase.ts

@@ -7,10 +7,7 @@ const isLocalDev = supabaseUrl.includes('localhost')
 // supabase-js hardcodes /auth/v1 as the auth path prefix, but GoTrue
 // serves at the root when accessed directly (no API gateway).
 // This custom fetch strips the prefix for local dev.
-function localGoTrueFetch(
-  input: RequestInfo | URL,
-  init?: RequestInit,
-): Promise<Response> {
+function localGoTrueFetch(input: RequestInfo | URL, init?: RequestInit): Promise<Response> {
   const url = input instanceof Request ? input.url : String(input)
   const rewritten = url.replace('/auth/v1/', '/')
   if (input instanceof Request) {
@@ -24,6 +21,10 @@ function createSupabaseClient(): SupabaseClient {
     return createClient('http://localhost:9999', 'stub-key')
   }
   return createClient(supabaseUrl, supabaseAnonKey, {
+    auth: {
+      autoRefreshToken: true,
+      persistSession: true,
+    },
     ...(isLocalDev && {
       global: { fetch: localGoTrueFetch },
     }),