fix(messages): address S2 review — partner key refresh + reactive me

- On a decrypt failure, refresh the partner's cached public key (and drop the
  stale derived conversation key) once per pass and retry. A passphrase reset
  rotates a keypair, so a partner who cached the old public key would otherwise
  encrypt undecryptable messages until a reload; the first failed decrypt now
  self-heals reads AND future sends. (pubCache was assumed set-once.)
- KeyGate reads has_password via a reactive useQuery(['me'], enabled:false)
  passive observer instead of a one-shot getQueryData, so the reset password
  field stays correct even if me resolves after mount.
- Document the remaining multi-device staleness (a reset on another device
  leaves this one 'ready' with the dead key) as a known limitation in useKeyState.

frontend/src/lib/e2ee.ts

@@ -185,6 +185,13 @@ export async function unlock(
   await installKey(userId, await importPrivate(pkcs8));
 }
 
+// Drop the cached derived conversation key for a partner. convKeyFor caches by partnerId alone, so
+// after a partner rotates their public key (a passphrase reset) the caller must call this before
+// re-deriving with the fresh key — otherwise the stale key would be returned.
+export function forgetConv(partnerId: number): void {
+  convKeys.delete(partnerId);
+}
+
 // --- per-conversation encryption ---
 async function convKeyFor(partnerId: number, partnerPubB64: string): Promise<CryptoKey> {
   const cached = convKeys.get(partnerId);