feat(titlebar): selectable window button styles + minimize toggle (#1083)

* feat(titlebar): selectable window button styles + minimize toggle

Custom title bar (Linux) gains a window-button style picker, mirroring
the seekbar style picker pattern. Six form-named styles: dots, dotsGlyph,
flat, pill, outline, glyph. All buttons now carry minimize/maximize/close
glyphs for clear, colour-blind-friendly iconography; dots reveal glyphs on
hover, dotsGlyph always shows them.

- New authStore state windowButtonStyle (default dots) + showMinimizeButton,
  with rehydrate validation falling back to dots on unknown values.
- WindowButtonPreview reuses the real .titlebar-btn classes for WYSIWYG tiles.
- Picker + minimize toggle render under the Custom title bar setting, gated
  on the toggle being on.
- Monochrome styles use --text-primary glyphs and stronger borders for
  contrast on dark themes.
- Dev-build grey marker scoped to the real title bar so previews show true
  colours.
- i18n keys in all 9 locales; setter and rehydrate tests.

* docs(changelog): window button styles (#1083)

src/store/authStoreRehydrate.ts

@@ -19,6 +19,7 @@ import type {
   LyricsSourceConfig,
   QueueDisplayMode,
   SeekbarStyle,
+  WindowButtonStyle,
 } from './authStoreTypes';
 import { migrateLegacyLastfm, sanitizeAccounts } from '../music-network';
 
@@ -93,6 +94,17 @@ export function computeAuthStoreRehydration(state: AuthState): Partial<AuthState
     ? {}
     : { seekbarStyle: 'truewave' as SeekbarStyle };
 
+  // Unknown / missing / tampered window-button style falls back to the
+  // default 'dots' so the title bar never renders an unstyled data-attr.
+  const VALID_WINDOW_BUTTON_STYLES = new Set<string>([
+    'dots', 'dotsGlyph', 'flat', 'pill', 'outline', 'glyph',
+  ]);
+  const windowButtonStyleMigrated = VALID_WINDOW_BUTTON_STYLES.has(
+    (state as { windowButtonStyle?: unknown }).windowButtonStyle as string,
+  )
+    ? {}
+    : { windowButtonStyle: 'dots' as WindowButtonStyle };
+
   // Garbage / null / undefined / missing key from a legacy or tampered persist
   // payload maps back to 'total' so the duration chip never receives an
   // unknown mode (would render an empty label).
@@ -236,6 +248,7 @@ export function computeAuthStoreRehydration(state: AuthState): Partial<AuthState
     ...youLyPlusMigrated,
     ...wheelSmoothOneTime,
     ...seekbarStyleMigrated,
+    ...windowButtonStyleMigrated,
     ...queueDurationDisplayModeMigrated,
     ...queueDisplayModeMigrated,
     ...linuxWaylandTextRenderProfileMigrated,