feat(settings): add "Square Corners" appearance toggle (#1215)

* feat(settings): add "Square Corners" appearance toggle

A Display toggle in Settings → Appearance → Visual Options that overrides
the active theme to render cards and cover art with square (non-rounded)
corners, for a sharper, boxy look.

Covered surfaces: grid cards (album/playlist/artist/song/because) and
their covers, detail-header heros (album/playlist/tracks), the Now
Playing / Radio view, the fullscreen player, the cover lightbox, the
queue panel cover, and the mini player.

Persisted in themeStore and driven by an `html[data-square-corners]`
attribute set in App.tsx — applied in both the main and mini-player
webviews (the mini player rehydrates the theme store on the shared
`storage` event). Strings added for all 12 locales.

* docs(changelog): add Square Corners to 1.50.0 CHANGELOG and What's New

CHANGELOG.md

@@ -9,6 +9,17 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 >
 
 
+## [1.50.0]
+
+## Added
+
+### Square corners — sharp-edged cards and covers
+
+**By [@Psychotoxical](https://github.com/Psychotoxical), PR [#1215](https://github.com/Psychotoxical/psysonic/pull/1215)**
+
+* New **Square Corners** toggle under **Settings → Appearance → Visual Options → Display** overrides the active theme to render cards and cover art with square, non-rounded corners. Covers album, playlist, artist and song cards, detail-page cover art, the Now Playing / Radio and fullscreen views, the cover lightbox, the queue cover, and the mini player. Off by default; buttons, inputs and dialogs keep the theme's corners.
+
+
 ## [1.49.0] - 2026-06-29
 
 ## Added