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>

docs/platform-notes.md

@@ -9,6 +9,7 @@ macOS is the primary target; Windows 11+ x64 and Linux x64 are **beta** targets
 | Concern | File | mac | win | linux |
 |---|---|---|---|---|
 | Window chrome | `src/bun/windows.ts chromeOptions` | hiddenInset + traffic-light offset, UI drag header | native titlebar, compact header | same as win |
+| Scrollbars | `ui/src/styles.css` (`::-webkit-scrollbar`) | custom square, always visible (styling them turns off overlay scrollbars) | custom square | custom square |
 | External editor launch | `src/bun/edits.ts launchEditor` | `open [-a app]` | `cmd /c start "" [app] file` | app directly / `xdg-open` |
 | Editor app picker | `rpc-handlers.ts pickEditorApp` | /Applications, `.app` | %ProgramFiles%, `.exe` | home, any |
 | Owner/group names | `fs/local.ts resolveId` | `id -nu` + `dscacheutil` | undefined → "—" | `id -nu` + `getent` |