Add configurable TUI themes

Hi, I updated the theme branch after syncing with current master.

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

Latest branch head:
c58ef0db

Compare:
https://github.com/1jehuang/jcode/compare/master...alecuba16:feature/themes-light-dark-config

Current state on master

Master already has important theme work now:

• Terminal background detection (dark / light / auto behavior)
• ThemeMode support in jcode-tui-style
• Buffer-level color adaptation with adapt_buffer_for_theme()
• This covers the automatic readable mode for light terminal backgrounds

So the branch is no longer trying to replace that work.

Current state of my branch

The branch now keeps the master behavior and builds on top of it:

• system theme uses the master native palette and keeps master terminal-background adaptation
• Explicit light, dark, and custom TOML themes are treated as final palettes
• Explicit themes skip adapt_buffer_for_theme() to avoid double color transformation
• /theme command to show and switch themes
• 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
• Background/input/markdown/header colors can be controlled by the theme palette
• Tests for built-in themes, custom theme loading, unsafe name rejection, and adaptation policy

Requested follow-up

Would this split make sense for the project?

• system = master behavior, terminal auto detection and buffer adaptation
• light / dark / custom = explicit user-selected palettes, no automatic buffer adaptation

I think this keeps the upstream automatic theme support, but still gives users a way to choose exact colors or add their own theme file.

Validation run

After rebasing on current master, I ran:

• cargo test -p jcode-tui-style theme -- --test-threads=1
• cargo test -p jcode-tui-style theme_mode -- --test-threads=1
• cargo check -p jcode-tui-style -p jcode-tui

All passed.

If this direction sounds good, I can keep the branch focused on the explicit theme switcher and custom theme file support, without changing the master auto-detection behavior.

Start by reviewing the feature/themes-light-dark-config branch and its master comparison, then run the listed jcode-tui-style theme tests and jcode-tui checks. Done means maintainers accept the system versus explicit-theme split and the validation commands continue to pass.