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/styles.css

@@ -1240,3 +1240,74 @@
   color: var(--t-3);
   font-weight: 500;
 }
+
+/* ─── Settings: game-folder field ─── */
+.folder-field {
+  display: inline-flex;
+  align-items: center;
+  gap: 8px;
+  width: 340px;
+  height: 36px;
+  padding: 0 4px 0 12px;
+  background: var(--bg-3);
+  border: 1px solid var(--bd-1);
+  border-radius: 8px;
+  transition: border-color .15s, background .15s;
+}
+.folder-field:hover { border-color: var(--bd-2); }
+.folder-field.is-unset {
+  border-color: color-mix(in srgb, var(--danger) 35%, var(--bd-1));
+  background: color-mix(in srgb, var(--danger) 6%, var(--bg-3));
+}
+.folder-field.is-unset:hover {
+  border-color: color-mix(in srgb, var(--danger) 55%, var(--bd-2));
+}
+.folder-field-icon {
+  display: inline-flex;
+  color: var(--t-3);
+  flex-shrink: 0;
+}
+.folder-field.is-unset .folder-field-icon { color: #f87171; }
+.folder-field-path {
+  flex: 1;
+  min-width: 0;
+  font-family: ui-monospace, "SF Mono", Menlo, Consolas, monospace;
+  font-size: 12px;
+  color: var(--t-1);
+  white-space: nowrap;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  direction: rtl;          /* truncate from the head so the leaf folder stays visible */
+  text-align: left;
+  unicode-bidi: plaintext; /* keep character order intact */
+}
+.folder-field-empty {
+  font-family: var(--font-ui);
+  font-size: 12.5px;
+  font-weight: 600;
+  color: #f87171;
+  letter-spacing: 0.1px;
+}
+.folder-field-btn {
+  flex-shrink: 0;
+  height: 28px;
+  padding: 0 12px;
+  border: 0;
+  border-radius: 6px;
+  background: rgba(255,255,255,0.06);
+  color: var(--t-1);
+  font: inherit;
+  font-size: 12.5px;
+  font-weight: 600;
+  letter-spacing: 0.1px;
+  cursor: pointer;
+  transition: background .15s, color .15s;
+}
+.folder-field-btn:hover { background: rgba(255,255,255,0.12); }
+.folder-field.is-unset .folder-field-btn {
+  background: color-mix(in srgb, var(--accent, #3b82f6) 85%, transparent);
+  color: #fff;
+}
+.folder-field.is-unset .folder-field-btn:hover {
+  background: var(--accent, #3b82f6);
+}