style(frontend): Prettier config + one-time repo-wide format pass

Prettier 3 (pinned), printWidth 100, double quotes — chosen to match the existing
hand-formatting and minimise reflow. This commit is ONLY the mechanical format pass
(`prettier --write`) plus the config/ignore/scripts, isolated so it never pollutes a
feature diff. Verified behaviour-neutral: typecheck, eslint (0 errors), and vitest
all green before and after. See .git-blame-ignore-revs — `git blame` skips this sha.

frontend/src/components/ChatThread.tsx

@@ -36,7 +36,7 @@ export default function ChatThread({
   const q = useLiveQuery<{ partner: MessageUser; items: Message[] }>(
     ["thread", partnerId],
     () => api.thread(partnerId),
-    { intervalMs: POLL_MS, enabled: !needsKey }
+    { intervalMs: POLL_MS, enabled: !needsKey },
   );
   const items = q.data?.items ?? [];
 
@@ -144,7 +144,7 @@ export default function ChatThread({
         ) : (
           items.map((m) => {
             const mine = m.sender_id === meId;
-            const text = m.kind === "system" ? m.body ?? "" : plain[m.id] ?? "…";
+            const text = m.kind === "system" ? (m.body ?? "") : (plain[m.id] ?? "…");
             return (
               <div key={m.id} className={`flex ${mine ? "justify-end" : "justify-start"}`}>
                 <div
@@ -165,7 +165,9 @@ export default function ChatThread({
       </div>
 
       {isSystem ? (
-        <div className="p-2 border-t border-border text-center text-xs text-muted">{t("messages.systemReadOnly")}</div>
+        <div className="p-2 border-t border-border text-center text-xs text-muted">
+          {t("messages.systemReadOnly")}
+        </div>
       ) : (
         <div className="flex items-end gap-2 p-2 border-t border-border">
           <textarea