Add dark/light theme support to the TUI

[ravshansbox]

Summary

The TUI/CLI agent (jcode) has no dark/light theme support. All colors are hardcoded RGB values tuned for dark terminals. The desktop app already supports System/Dark/Light, so the TUI is the gap.

Current state

• TUI: Fixed palette in crates/jcode-tui-style/src/theme.rs (e.g. user_color() -> rgb(138, 180, 248)). No theme config option, no terminal-background detection (no COLORFGBG/luminance probing), no light variant.
• Desktop: crates/jcode-desktop/src/desktop_ui_engine.rs defines enum ThemeMode { System, Dark, Light } with DesktopTheme::for_preferences() resolving System against the OS setting, plus explicit light()/dark() palettes stored in UiPreferences (defaults to System).

Proposed work

• Introduce a theme abstraction in jcode-tui-style instead of constant color functions (e.g. a Theme struct with named roles).
• Provide built-in dark and light palettes.
• Add a config option (e.g. theme = "system" | "dark" | "light" in the config file).
• Optionally auto-detect terminal background via COLORFGBG / OSC 11 query for system mode.
• Keep the existing dark palette as the default to avoid visual regressions.

Acceptance criteria

• Users can select dark or light themes via config.
• A readable light palette exists for light terminals.
• Default behavior is unchanged for current users.