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)

CHANGELOG.md

@@ -131,6 +131,15 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 * Play counts are unchanged — still driven by the existing scrobble path. Servers without the extension keep the previous now-playing behaviour.
 
 
+
+### Title bar — selectable window button styles
+
+**By [@Psychotoxical](https://github.com/Psychotoxical), PR [#1083](https://github.com/Psychotoxical/psysonic/pull/1083), suggested by [@PHLAK](https://github.com/PHLAK)**
+
+* The Linux custom title bar gets a **window button style** picker in **Settings → Appearance → Custom title bar** — choose between dots, dots with icons, flat, pill, outline, and minimal looks.
+* All styles now carry minimize/maximize/close icons for clear, colour-blind-friendly buttons, and an optional toggle hides the minimize button (maximize and close only).
+
+
 ## Changed
 
 ### Dependencies — npm and Rust refresh