docs(design): move game folder selection into settings

Move the design contract for choosing the library game folder out of the top
bar and into Settings > Library. The launcher chrome now reserves the right
edge for the kebab/menu controls, while Settings owns the required folder path
and its set/unset states.

The reference mock now persists `gameFolder: string | null` instead of a
boolean, adds an exercisable GameFolderField, and includes matching CSS so the
prototype reflects the documented interaction. This is still design/reference
only; no runtime Tauri settings code changes here.

Test Plan:
- git diff --cached --check

Refs: none

design/design_reference/launcher.jsx

@@ -1,8 +1,6 @@
 // launcher.jsx — composes top bar + grid into a complete launcher screen
 // Comes in two chrome variants: 'single' (one-row) and 'two' (two-row).
 
-const DIR_PATH = '/home/pfs/Desktop/eti_games_AFTER_LAN_2025';
-
 function applyFilterAndSort(games, filter, sort, query) {
   let g = filterGames(games, filter);
   if (query.trim()) {
@@ -28,7 +26,7 @@ function Launcher({
   initialOpenGame = null,
   initialSettingsOpen = false,
 }) {
-  const { density, aspect, accent, bg, gameFolderSet } = tweaks;
+  const { density, aspect, accent, bg } = tweaks;
   const [filter, setFilter] = useState(initialFilter);
   const [sort, setSort] = useState(initialSort);
   const [query, setQuery] = useState(initialQuery);
@@ -68,7 +66,6 @@ function Launcher({
               <SortMenu value={sort} onChange={setSort} accent={accent}/>
             </div>
             <div className="topbar-right-trail">
-              <DirectoryButton path={gameFolderSet === false ? null : DIR_PATH}/>
               <KebabMenu items={menuItems}/>
             </div>
           </div>
@@ -80,7 +77,6 @@ function Launcher({
               <div className="brand-mark" style={{ background: accent }}>S</div>
               <div className="brand-name">SoftLAN <span className="brand-name-soft">Launcher</span></div>
             </div>
-            <DirectoryButton path={gameFolderSet === false ? null : DIR_PATH}/>
             <div className="topbar-row1-right">
               <StorageMeter accent={accent}/>
               <KebabMenu items={menuItems}/>