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/components/ChatThread.tsx

@@ -8,7 +8,7 @@ import { useScrollFade } from "../lib/useScrollFade";
 import { LoadingState, StateMessage } from "./QueryState";
 import { relativeTime } from "../lib/format";
 import { notify } from "../lib/notifications";
-import { partnerPub, POLL_MS, useKeyState } from "../lib/messaging";
+import { partnerPub, POLL_MS, refreshPartnerPub, useKeyState } from "../lib/messaging";
 import * as e2ee from "../lib/e2ee";
 import KeyGate from "./KeyGate";
 
@@ -49,14 +49,28 @@ export default function ChatThread({
     let cancelled = false;
     (async () => {
       const out: Record<number, string> = {};
+      let pub = await partnerPub(partnerId);
+      let refreshed = false;
       for (const m of q.data?.items ?? []) {
-        if (m.ciphertext && m.iv) {
-          try {
-            const pub = await partnerPub(partnerId);
-            out[m.id] = await e2ee.decryptFrom(partnerId, pub, m.ciphertext, m.iv);
-          } catch {
-            out[m.id] = "⚠ " + t("messages.cantDecrypt");
+        if (!m.ciphertext || !m.iv) continue;
+        try {
+          out[m.id] = await e2ee.decryptFrom(partnerId, pub, m.ciphertext, m.iv);
+        } catch {
+          // A decrypt failure can mean the partner rotated their keypair (a passphrase reset), so
+          // this device's cached public key is stale. Refresh it ONCE per pass and retry — that
+          // also re-derives future sends under the current key. A genuinely bad message still ends
+          // up "can't decrypt".
+          if (!refreshed) {
+            refreshed = true;
+            try {
+              pub = await refreshPartnerPub(partnerId);
+              out[m.id] = await e2ee.decryptFrom(partnerId, pub, m.ciphertext, m.iv);
+              continue;
+            } catch {
+              /* fall through to the error label below */
+            }
           }
+          out[m.id] = "⚠ " + t("messages.cantDecrypt");
         }
       }
       if (!cancelled) setPlain(out);