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/ChannelLayoutGrid.tsx

@@ -49,7 +49,9 @@ export default function ChannelLayoutGrid<T>({
   const meta = columns.filter((c) => !c.cardPrimary && !slotted.has(c.key));
 
   const renderCard = (row: T) => (
-    <div className={`glass-card rounded-xl p-3 h-full flex flex-col gap-2 ${rowClassName?.(row) ?? ""}`}>
+    <div
+      className={`glass-card rounded-xl p-3 h-full flex flex-col gap-2 ${rowClassName?.(row) ?? ""}`}
+    >
       <div className="flex items-start justify-between gap-2">
         <div className="min-w-0 flex-1 font-medium">
           {primary.map((col) => (