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/i18n/locales/en/plex.json

@@ -232,4 +232,4 @@
     "count_other": "{{count}} titles",
     "remove": "Remove from filter"
   }
-}
+}

frontend/src/i18n/locales/hu/plex.json

@@ -232,4 +232,4 @@
     "count_other": "{{count}} cím",
     "remove": "Eltávolítás a szűrőből"
   }
-}
+}