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/en/settings.ts

@@ -231,6 +231,16 @@ export const settings = {
   discordRichPresenceNotice: 'Heads up: this is Psysonic\'s built-in Discord Rich Presence. If you\'d rather use the official Navidrome Discord Rich Presence plugin, leave this switched off and enable "Show in Now Playing" further down this page instead.',
   useCustomTitlebar: 'Custom title bar',
   useCustomTitlebarDesc: 'Replace the system title bar with a built-in one that matches the app theme. Disable to use the native GNOME/GTK title bar.',
+  windowButtonStyle: 'Window buttons',
+  windowButtonStyleDesc: 'Pick the look of the minimize, maximize and close buttons.',
+  windowButtonsDots: 'Dots',
+  windowButtonsDotsGlyph: 'Dots + icons',
+  windowButtonsFlat: 'Flat',
+  windowButtonsPill: 'Pill',
+  windowButtonsOutline: 'Outline',
+  windowButtonsGlyph: 'Minimal',
+  showMinimizeButton: 'Show minimize button',
+  showMinimizeButtonDesc: 'Turn off to show only the maximize and close buttons.',
   linuxWebkitSmoothScroll: 'Smooth wheel (Linux)',
   linuxWebkitSmoothScrollDesc: 'On: inertial scroll. Off: line-by-line, GTK-style.',
   linuxWebkitInputForceRepaint: 'Repaint inputs on focus (Linux)',