Phase 16: Petal authenticates for itself

Petal is now an OIDC client in its own right rather than trusting a header
from the proxy. The Phase-0 Resolver seam was the only integration point:
main.go picks the session store when Authentik is configured and the static
local user otherwise, and no handler or query moved for either.

internal/auth gains three pieces. session.go issues an opaque cookie token
and stores only its SHA-256, so a database copy yields nothing usable; the
30-day expiry slides on every request, throttled to one write an hour, and
logout deletes the row rather than just the cookie. oidc.go runs the
authorization-code flow with state, nonce and PKCE, and discovers the
provider lazily and on retry — an Authentik outage should block new logins
without stopping Petal booting or invalidating live sessions. users.go
provisions accounts from the token's claims and gates them on an allowlist
that matches emails as well as subject ids, since a subject is an opaque
uuid that doesn't exist until someone has already logged in once.

Migration 0010 lands sessions, images and users.pair_lang together. The
images table closes the capability-URL hole the Phase-0 audit flagged: a
hash was previously enough to fetch anyone's picture. Rows are keyed
(name, user_id) so one file can have several owners and deduplication
survives; a stranger gets 404 rather than 403, the cache header drops to
private, and files already on disk are claimed at startup or every image
already pasted into a document would 404.

On the frontend a single 401 interceptor feeds a warm bilingual sign-in
overlay, drawn over a still-visible editor because nothing has been taken
away. Behind it is the part that matters: a save that comes back 401
stashes its body to localStorage before anything else and stops the
auto-save loop, and reopening that document after signing in merges the
draft back and saves it. An expired session must not cost writing.

Writing the round-trip test against a stub identity provider turned up a
real bug: the one-shot state/nonce/PKCE cookies were cleared in a defer,
which runs after the redirect has written the response header, so the
clearing Set-Cookie was silently dropped and they lingered for their full
ten minutes.

Also swaps the emoji favicon for a drawn sakura, which renders as Petal's
own rose palette everywhere instead of whatever each platform's font
decides, and doubles as the app tile in Authentik.

Migration 0010 verified against a VACUUM INTO copy of the live millenia
database: counts intact, FTS still matching, the one existing image
claimed.

Claude-Session: https://claude.ai/code/session_016y6gyuHkQXPiEuW8RGQyua

web/src/components/StatusBar/StatusBar.tsx

@@ -26,6 +26,9 @@ const SAVE_LABEL: Record<SaveStatus, string> = {
   saving: 'Saving…',
   saved: 'Saved just now',
   error: "Couldn't save",
+  // The session lapsed. Say where the writing is, not what failed — it's safe
+  // on this device and goes up the moment she signs back in.
+  'signed-out': '已保存在本机 · Kept on this device',
 }
 
 // StatusBar is the slim footer: word count on the left, save state and the