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/components.jsx

@@ -524,6 +524,31 @@ function SegmentedRadio({ value, options, onChange, accent }) {
   );
 }
 
+function GameFolderField({ value, onChange, accent }) {
+  const isSet = !!(value && value.trim());
+  const handleChange = () => {
+    // In production: open native folder picker via Tauri.
+    // For the prototype, prompt for a path so the field is exercisable.
+    const next = window.prompt('Game folder path (leave empty to clear)', value || '');
+    if (next == null) return;
+    onChange(next.trim());
+  };
+  return (
+    <div className={`folder-field ${isSet ? 'is-set' : 'is-unset'}`}
+         style={{ '--accent': accent }}>
+      <span className="folder-field-icon" aria-hidden="true"><Icon.folder/></span>
+      <div className="folder-field-path" title={isSet ? value : 'No folder selected'}>
+        {isSet
+          ? <bdi>{value}</bdi>
+          : <span className="folder-field-empty">Not set</span>}
+      </div>
+      <button type="button" className="folder-field-btn" onClick={handleChange}>
+        {isSet ? 'Change\u2026' : 'Choose\u2026'}
+      </button>
+    </div>
+  );
+}
+
 function ColorSwatchPicker({ value, options, onChange }) {
   return (
     <div className="swatch-row">
@@ -582,6 +607,11 @@ function SettingsDialog({ settings, onChange, onClose }) {
           </div>
           <div className="settings-section">
             <div className="settings-section-title">Library</div>
+            <SettingsRow label="Game folder" hint="Parent directory where games are downloaded and installed">
+              <GameFolderField value={settings.gameFolder}
+                               onChange={(v) => onChange('gameFolder', v)}
+                               accent={settings.accent}/>
+            </SettingsRow>
             <SettingsRow label="Grid density" hint="How tightly cards are packed">
               <SegmentedRadio value={settings.density}
                               options={SETTING_OPTIONS.density}