fix(v2): SSO buttons no longer vanish on a transient providers-fetch blip

[lilyshen0722]

Root cause of the recurring "I can't see the SSO option, sometimes after a while I can" reports. V2OAuthButtons did:

axios.get('/api/auth/oauth/providers')
  .then(res => setProviders(res.data?.providers ?? []))
  .catch(() => setProviders([]));   // ANY failure → empty
if (!providers.length) return null; // empty → nothing, no retry

So a single failed/slow fetch — a deploy blip, a spot reclaim, or the user's own network hiccup — set providers to empty and hid the buttons with no retry, until a manual reload ("after a while I can see it"). It turned every brief transient into a sticky, visible broken-login state, and it's specifically the SSO buttons because they're the only login element gated on a fragile fetch. (The underlying backend blips themselves are separately fixed by #676/#679; this is the frontend amplifier.)

Fix:

• Seed state from a localStorage last-known-good cache → repeat visitors see buttons instantly, and a transient failure falls back to the cached list instead of blanking.
• Retry the fetch 3× with backoff.
• Only clear on a successful empty response (genuine self-hosted-without-OAuth) — never on a network error.
• Cancel the pending retry timer on unmount (no stray request after navigating away).

Tests: added fall-back-to-cache-on-failure and retry-until-recovery; 6/6 in V2OAuth.test.tsx, 40/40 v2 suite green.

Zero-downtime deployable.

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https://claude.ai/code/session_01MnRCAFgjrrGZxo9VRCmCm9