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/lib/messagesSocket.ts

@@ -45,7 +45,11 @@ function open() {
     try {
       const data = JSON.parse(ev.data);
       if (data?.type === "message" && data.message) {
-        const e: IncomingMessage = { message: data.message as Message, from: data.from, to: data.to };
+        const e: IncomingMessage = {
+          message: data.message as Message,
+          from: data.from,
+          to: data.to,
+        };
         for (const h of handlers) h(e);
       } else if (data?.type === "unread") {
         // Sent to a user's OTHER tabs when they read a thread elsewhere, so the badge updates live.