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/VirtualGrid.tsx

@@ -1,4 +1,12 @@
-import { Fragment, useEffect, useLayoutEffect, useMemo, useRef, useState, type ReactNode } from "react";
+import {
+  Fragment,
+  useEffect,
+  useLayoutEffect,
+  useMemo,
+  useRef,
+  useState,
+  type ReactNode,
+} from "react";
 import { useVirtualizer } from "@tanstack/react-virtual";
 
 // The virtualization engine, lifted out of VirtualFeed so any long client- or server-side list can
@@ -109,7 +117,7 @@ export default function VirtualGrid<T>({
       const cols = columnsFor(minCol, w);
       setColCount(cols);
       setItemWidth(
-        minCol == null ? Math.min(w, maxCol ?? w) : Math.floor((w - GRID_GAP * (cols - 1)) / cols)
+        minCol == null ? Math.min(w, maxCol ?? w) : Math.floor((w - GRID_GAP * (cols - 1)) / cols),
       );
     };
     measure();