Adopt Oxide's color scheme and a square design

The palette now follows Oxide's design system (docs.oxide.computer): a cool
neutral scale at hue 260 with almost no chroma, opaque strokes, and Oxide's
green as the only accent. The accent-color option is gone: selection, the
cursor row, focus rings and the active-panel line all read green.

The file panel's header strip now steps toward the list, one shade per row
(session strip, path bar, column headers), derived by color-mix so every
theme, AMOLED included, follows on its own.

Everything is square: every rounded utility and the radius tokens are gone,
and checkboxes, radios, selects, number spinners and scrollbars are
restyled to match (square scrollbars are always visible on macOS, the cost
of styling them).

Co-Authored-By: Claude <noreply@anthropic.com>

README.md

@@ -55,7 +55,7 @@ Dense reference material lives in [docs/](docs/): [architecture](docs/architectu
 | Runtime    | Bun (main process, all SSH/SFTP + file IO)                    |
 | Desktop    | Electrobun 1.18 (native webview, typed RPC bridge)            |
 | Frontend   | Angular 22 (standalone, signals, zoneless) in `ui/`           |
-| Styling    | Tailwind CSS v4, shadcn-style tokens (no component library)   |
+| Styling    | Tailwind CSS v4, Oxide-derived tokens (no component library)  |
 | Icons      | @ng-icons/tdesign-icons                                       |
 | SSH/SFTP   | `ssh2` (pure-JS under Bun)                                    |