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/authStore.settings.test.ts

@@ -58,6 +58,8 @@ describe('trivial pass-through setters', () => {
     ['setDiscordRichPresence', 'discordRichPresence', true],
     ['setEnableBandsintown', 'enableBandsintown', true],
     ['setUseCustomTitlebar', 'useCustomTitlebar', true],
+    ['setWindowButtonStyle', 'windowButtonStyle', 'flat'],
+    ['setShowMinimizeButton', 'showMinimizeButton', false],
     ['setPreloadMiniPlayer', 'preloadMiniPlayer', true],
     ['setLinuxWebkitKineticScroll', 'linuxWebkitKineticScroll', false],
     ['setLinuxWaylandTextRenderProfile', 'linuxWaylandTextRenderProfile', 'gpu'],