feat(ui): redesign game-folder button as icon + label + dot

Implements the v2 design's game-folder button in the Tauri launcher.
The previous control squeezed the full game-directory path into the
top-bar button as truncated monospace (e.g.
`…s/Desktop/eti_games_AFTER_LAN_2025`). In practice the leading
ellipsis rarely showed the meaningful part of the path, ate
horizontal space the new 3-zone top bar needs for its primary
controls, and competed with the filter / search / sort cluster for
attention.

The button now communicates the *state* of the configuration at a
glance — an icon + short label + colored status dot — while the full
path moves into the native tooltip and `aria-label`, where it stays
one mouseover (and screen-reader friendly) away.

Two visual states, both 36 px tall and sharing the surface of the
other top-bar controls:

- **Set & valid** (`.dirbtn-set`) — label `Game folder`, green dot
  (`--ok`) with a soft glow, default border, tooltip = full path.
- **Not set / invalid** (`.dirbtn-unset`) — label `Set game folder`,
  red dot (`--danger`) with a soft glow, a red-tinted border, and a
  faint red wash on hover so the bad state reads as "this is what
  you need to fix". Tooltip = `Please select a game folder`.

`DirectoryButton` now takes `path: string | null` and picks the
state from `!!(path && path.trim())`. The `aria-label` carries the
full state in words (`Game folder: <path>` / `Set game folder`) so
screen readers don't have to interpret the colored dot.

Note: in the current `MainWindow`, `gameDir` is gated upstream — if
no directory is selected, `NoDirectoryState` is shown instead of the
top bar, so the unset state will only surface here if we later
validate disk existence and clear `gameDir`. The button accepts a
nullable path anyway, so it's ready when that check lands.

`truncatePath` in `lib/format.ts` was the only caller-less helper
left behind and is removed.

Test Plan

- `npx tsc --noEmit` from the frontend crate — clean.
- `just frontend-test` — passes.
- Manual: `just run`, pick a valid game directory, and confirm the
  top-bar button reads `Game folder` with a green dot; hovering it
  reveals the full path in the OS tooltip. Inspect the DOM and
  confirm `aria-label` reads `Game folder: <path>`. (The unset
  variant currently isn't surfaced by `MainWindow`; eyeball it via
  DevTools by toggling the `dirbtn-set` class to `dirbtn-unset`.)

crates/lanspread-tauri-deno-ts/src/lib/format.ts

@@ -19,10 +19,6 @@ export const formatEtiVersion = (raw: string | undefined): string => {
     return raw;
 };
 
-/** Truncate a path with a leading ellipsis when it exceeds the limit. */
-export const truncatePath = (path: string, max = 36): string =>
-    path.length > max ? `…${path.slice(-(max - 1))}` : path;
-
 export const formatPlayers = (max?: number): string => {
     if (!max || max <= 0) return '—';
     return max === 1 ? '1' : `1–${max}`;