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/SoftLAN Launcher.html

@@ -36,9 +36,9 @@ const TWEAK_DEFAULTS = /*EDITMODE-BEGIN*/{
   "density": "normal",
   "aspect": "square",
   "bg": "gradient",
-  "username": "d",
+  "username": "ddidderr",
   "language": "en",
-  "gameFolderSet": true
+  "gameFolder": "\/some\/folder\/to\/games"
 }/*EDITMODE-END*/;
 
 const ACCENTS = ['#3b82f6', '#22d3ee', '#a855f7', '#22c55e', '#f59e0b', '#ef4444'];
@@ -116,8 +116,8 @@ function App() {
                     onChange={(v) => setTweak('aspect', v)}/>
 
         <TweakSection label="Library"/>
-        <TweakToggle label="Game folder set" value={t.gameFolderSet}
-                     onChange={(v) => setTweak('gameFolderSet', v)}/>
+        <TweakText label="Game folder" value={t.gameFolder}
+                   onChange={(v) => setTweak('gameFolder', v)}/>
       </TweaksPanel>
     </React.Fragment>
   );