Add configurable TUI themes

[alecuba16]

Hi, I prepared a theme feature branch in my fork, because the contributions are closed.

Branch:
https://github.com/alecuba16/jcode/tree/feature/themes-light-dark-config

Compare:
master...alecuba16:feature/themes-light-dark-config

Reason:

• The TUI only had the existing/default color behavior.
• The readable/light theme had contrast issues in practice: white was too bright, pink was too light, and light mode could inherit a dark terminal background when the frame was cleared.
• There was no dark built-in theme.
• There was no easy way to switch themes or add custom themes from config.

What the branch adds:

• Built-in light, dark, and system themes. Light is the current default/builtin theme.
• /theme command with status, light, dark, system, and custom theme names.
• display.theme config persistence.
• Custom theme loading from ~/.jcode/themes/<name>.toml.
• Safe custom theme names (A-Z, a-z, 0-9, _, -) to avoid path traversal / drive-prefix issues.
• Frame background handling so light mode clears with an explicit light background instead of inheriting a dark terminal background.
• Docs for custom themes in docs/themes.md.
• Tests for built-in/custom theme loading, path rejection, and light frame background behavior.

Validation run:

• cargo fmt --package jcode-tui-style --package jcode-tui-render --package jcode-tui
• cargo test -p jcode-tui-style
• cargo test -p jcode-tui-render clear_area_uses_active_theme_frame_background
• cargo check -p jcode-tui-style -p jcode-tui
• cargo build -p jcode-tui

If pull requests from forks are not enabled, could you please take a look at the branch and integrate it if it fits the project?

[1jehuang]

Triage: thanks for the thorough branch and validation notes. Theming is a real gap (light-terminal contrast issues are a recurring theme, see also #415 dark/light theme support - these should probably be decided together). Marked needs-decision because it introduces user-facing surface area (/theme command, display.theme config, ~/.jcode/themes/*.toml format) that we want to commit to once, not churn. The safe-name handling and explicit light frame background look like the right calls. Will review the compare branch alongside #415.

[alecuba16]

Thanks for taking a look! No problem , if there's anything I can do to improve the quality, just let me know.