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/locales/nl/settings.ts

@@ -611,6 +611,16 @@ export const settings = {
   waveformCacheClearFailed: 'Waveform-cache wissen mislukt.',
   useCustomTitlebar: 'Aangepaste titelbalk',
   useCustomTitlebarDesc: 'Vervang de systeemtitelbalk door een ingebouwde balk die bij het app-thema past. Schakel uit om de native GNOME/GTK-titelbalk te gebruiken.',
+  windowButtonStyle: 'Vensterknoppen',
+  windowButtonStyleDesc: 'Kies het uiterlijk van de knoppen minimaliseren, maximaliseren en sluiten.',
+  windowButtonsDots: 'Stippen',
+  windowButtonsDotsGlyph: 'Stippen + iconen',
+  windowButtonsFlat: 'Plat',
+  windowButtonsPill: 'Pil',
+  windowButtonsOutline: 'Omtrek',
+  windowButtonsGlyph: 'Minimaal',
+  showMinimizeButton: 'Minimaliseerknop tonen',
+  showMinimizeButtonDesc: 'Schakel uit om alleen maximaliseren en sluiten te tonen.',
   analyticsStrategyTitle: 'Analysestrategieën',
   analyticsStrategyDesc: 'Kies de analysemethode voor de bibliotheek.',
   analyticsStrategyLabel: 'Strategie',