feat(web-ui): sync theme from native app to web UI

Move shared theme definitions (38 themes, ThemeKind, ThemePalette) from
arbor-gui into arbor-core so both the GUI and httpd can use them.

Add GET /api/v1/config/theme endpoint that reads the theme slug from
~/.config/arbor/config.toml and returns the full palette as hex strings.

The web UI fetches the theme on startup and polls every 5 seconds,
applying all palette colors as CSS custom properties. The xterm.js
terminal theme is also updated with proper ANSI color mappings for
both light and dark themes, including contrast-safe values for light
backgrounds.

Author: penso

README.md

@@ -82,7 +82,7 @@ It gives you one place to manage repositories, issue-driven worktrees, embedded
 - Three-pane layout across `arbor-gui` and `arbor-web-ui`
 - Command palette coverage for actions, repos, worktrees, issues, presets, and task templates
 - Resizable panes, collapsible sidebar, desktop notifications
-- More than twenty-five themes, including Omarchy defaults and white VS Code inspired themes
+- 38 themes shared between the desktop app and web UI, with live sync when switching themes
 - Branch-aware window titles and daemon / rate-limit status polish
 - TOML config at `~/.config/arbor/config.toml` with hot reload
 

crates/arbor-core/src/lib.rs

@@ -11,6 +11,7 @@ pub mod remote;
 pub mod repo_config;
 pub mod session;
 pub mod task;
+pub mod theme;
 pub mod worktree;
 pub mod worktree_name;
 pub mod worktree_scripts;