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/linkify.tsx

@@ -26,7 +26,7 @@ const PLATFORMS: Record<string, string> = {
 //         (to the platform's profile URL); the "<platform>: " label stays as text. Bare @handles
 //         with no platform, and #hashtags, are left plain.
 const URL_SOURCE =
-  '(?:https?://|www\\.)[^\\s<]+[^\\s<.,!?;:)\\]}"\']|[a-z0-9-]+(?:\\.[a-z0-9-]+)*\\.[a-z]{2,}/[^\\s<]*[^\\s<.,!?;:)\\]}"\']';
+  "(?:https?://|www\\.)[^\\s<]+[^\\s<.,!?;:)\\]}\"']|[a-z0-9-]+(?:\\.[a-z0-9-]+)*\\.[a-z]{2,}/[^\\s<]*[^\\s<.,!?;:)\\]}\"']";
 const MENTION_SOURCE =
   "\\b(" +
   Object.keys(PLATFORMS).join("|") +