docs(design): redesign game-folder button as icon + label + dot

The previous design squeezed the full game-directory path into the
top-bar button as truncated `ui-monospace` (e.g.
`…s/Desktop/eti_games_AFTER_LAN_2025`). In practice the leading-ellipsis
truncation rarely showed the meaningful part of the path on real-world
configurations, ate horizontal space the new 3-zone top bar needs for
its actual primary controls, and competed with the filter / search /
sort cluster for attention.

Replace the inline path with an icon + short label + colored status
dot. The full path moves into the tooltip and `aria-label`, where it's
still one mouseover (and screen-reader friendly) away. The button now
communicates the *state* of the configuration at a glance — which is
what users actually need.

Two visual states, both 36 px tall with the same surface as the other
top-bar controls:

- **Set & valid** — label `Game folder`, green dot (`--ok`) with a
  soft glow, default border, tooltip = full path.
- **Not set / invalid** — 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`.

"Invalid" (a path is stored but doesn't exist on disk) is collapsed
into the same visual state as "not set" — the user's required action
is identical (open the picker, pick a folder), so a third state
isn't worth the visual budget yet. If we later want to surface a
*last-known* path so the user can re-attach an external drive,
introduce a distinct missing state then.

Implementation notes:

- `DirectoryButton` now takes a single `path: string | null` prop and
  picks state from `!!(path && path.trim())`. Children are
  `Icon.folder`, the label, and an 8 px `.dirbtn-status-dot` sibling
  — the dot is an inline flex sibling, not a corner badge, because
  the button is now wider than tall and a corner pin would feel
  misplaced.
- `.dirbtn` is `inline-flex` with `padding: 0 14px 0 12px`, gap 8 px,
  `white-space: nowrap`, and `flex-shrink: 0`. The `max-width: 360px`
  cap from the path-truncation era is gone — the button is now
  intrinsically sized.
- Dot glow uses `box-shadow: 0 0 6px color-mix(...)` so it still
  reads through the launcher's translucent top-bar background.
- The Tweaks panel grows a dev-only `Game folder set` toggle (under
  the new *Library* section) that flips a `gameFolderSet` flag wired
  into the Launcher, so reviewers can see both states without
  fiddling with real filesystem state. The README explicitly calls
  this out as **dev-only** — production state comes from the
  settings store, not a user-facing toggle.

The README gains a new *Game-folder button* section with the full
spec, a state table, and a rationale paragraph; the "Changes since v2"
list and the interactions list are updated to reflect the new label
and behavior.

Test Plan

- Open `design_reference/SoftLAN Launcher.html` and locate the Tweaks
  panel's *Library → Game folder set* toggle.
  - With the toggle **on**: the top-bar button shows `Game folder`
    with a green dot; hovering the button reveals the full mock path
    in the native tooltip.
  - With the toggle **off**: the label switches to `Set game folder`,
    the dot turns red, the border picks up a red tint, and hovering
    the button reveals `Please select a game folder`.
- Inspect the button with a screen reader / DevTools accessibility
  pane: the `aria-label` should read `Game folder: <path>` when set
  and `Set game folder` when unset.

design/design_reference/SoftLAN Launcher.html

@@ -37,7 +37,8 @@ const TWEAK_DEFAULTS = /*EDITMODE-BEGIN*/{
   "aspect": "square",
   "bg": "gradient",
   "username": "d",
-  "language": "en"
+  "language": "en",
+  "gameFolderSet": true
 }/*EDITMODE-END*/;
 
 const ACCENTS = ['#3b82f6', '#22d3ee', '#a855f7', '#22c55e', '#f59e0b', '#ef4444'];
@@ -113,6 +114,10 @@ function App() {
         <TweakRadio label="Cover aspect" value={t.aspect}
                     options={['box', 'square', 'banner']}
                     onChange={(v) => setTweak('aspect', v)}/>
+
+        <TweakSection label="Library"/>
+        <TweakToggle label="Game folder set" value={t.gameFolderSet}
+                     onChange={(v) => setTweak('gameFolderSet', v)}/>
       </TweaksPanel>
     </React.Fragment>
   );