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/components/topbar/DirectoryButton.tsx

@@ -1,17 +1,27 @@
 import { Icon } from '../Icon';
-import { truncatePath } from '../../lib/format';
 
 interface Props {
-    path: string;
+    path: string | null;
     onClick: () => void;
 }
 
-export const DirectoryButton = ({ path, onClick }: Props) => (
-    <button className="dirbtn" type="button" title={path || 'Choose a game directory'} onClick={onClick}>
-        <Icon.folder />
-        <span className="dirbtn-label">Game directory</span>
-        <span className="dirbtn-path">
-            {path ? truncatePath(path) : 'choose…'}
-        </span>
-    </button>
-);
+export const DirectoryButton = ({ path, onClick }: Props) => {
+    const isSet = !!(path && path.trim());
+    const label = isSet ? 'Game folder' : 'Set game folder';
+    const tooltip = isSet ? (path as string) : 'Please select a game folder';
+    const ariaLabel = isSet ? `Game folder: ${path}` : 'Set game folder';
+
+    return (
+        <button
+            type="button"
+            className={`dirbtn ${isSet ? 'dirbtn-set' : 'dirbtn-unset'}`}
+            title={tooltip}
+            aria-label={ariaLabel}
+            onClick={onClick}
+        >
+            <Icon.folder />
+            <span className="dirbtn-label">{label}</span>
+            <span className="dirbtn-status-dot" aria-hidden="true" />
+        </button>
+    );
+};