Settings: a settings window, its sections, a search, and what fills them

SettingsLayout lists sections down the side and shows the open one, the
list going above the page in a narrow box; SettingsSection and
SettingsRow title and hold settings, a row's control wrapping under its
name. SettingsSearch finds a setting by its name or words and opens its
section. ThemeSelector, AccentColorPicker, FontSizeControl and
DensitySelector set the look; KeyboardShortcutsList finds shortcuts by
every word; ProxySettings, PrivacySettings, NotificationSettings,
StartupSettings, StorageSettings with its clear, ImportExportSettings,
ResetToDefault, FeatureFlags and DeveloperModeToggle fill the rest.
Every handler a control needs is required and fails loud when missing.
ColorPalette wraps its swatches in a row capped at its columns, so it
keeps their size in a box sized by its content, and its swatches carry
the palette's id. ThemeSwitcher points here: src/theme sits under the
chapters that draw with buttons.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

AGENTS.md

@@ -193,3 +193,4 @@ Ely GPUI Component. A component library for GPUI, in light and dark.
 - 2026-09-25: SystemNotification is blocked. An unbundled app has no notification center (probed on macOS 27.2), and gpui 0.2.2 has no API.
 - 2026-09-25: `unexpected_cfgs` declares `feature = "cargo-clippy"`, which the objc 0.2 macros test.
 - 2026-09-26: Terminal proven with `alacritty_terminal` 0.26: its grid, parser and pseudo-terminal, drawn by Ely. A replayed grid serves recorded output and tests.
+- 2026-09-27: ThemeSwitcher is `settings::ThemeSelector`: `src/theme` sits under every chapter and cannot draw with buttons.