Feature: /theme slash command and custom theme palettes

[alecuba16]

Summary

Add a /theme slash command to the TUI for switching between built-in and custom color themes, plus support for custom theme palettes defined as TOML files under ~/.jcode/themes/<name>.toml.

Problem

jcode ships with auto/dark/light theme detection, but users have no way to:

1. Switch theme from inside the TUI without editing config and restarting.
2. Define a custom color palette (e.g. a branded or accessibility-oriented scheme).

Proposal

• /theme slash command cycles / selects themes (auto, dark, light, or a custom theme file name).
• Custom themes live in ~/.jcode/themes/<name>.toml and define a palette of named colors.
• Themes with an explicit background color fill Color::Reset cells so the app is readable regardless of the terminal default background.
• Custom themes that declare their own palette opt out of the terminal-adaptation (luminance-flip) pass.

Implementation

A working implementation is ready for review on the feature/themes-light-dark-config branch (rebased onto latest master, 699 insertions across 20 files, jcode-config-types + jcode-tui-style + jcode-tui build, theme + tui-style tests pass).

@1jehuang would be great if you could take a look.

[alecuba16]

@1jehuang the implementation of the /theme slash command and custom theme palettes is ready for review on the feature/themes-light-dark-config branch. It has been rebased onto the latest master and validated (theme + tui-style tests pass). Would be great if you could take a look.

[daviskathy6643]

契约测试有没有上 ✨🙌

[alecuba16]

Thanks for the question! Contract tests have now been added to the feature/themes-light-dark-config branch. They verify the public API contract of the theme system:

1. Round-trip contract: set_theme(name) → active_theme_name() == name — every builtin theme (system, light, dark) round-trips correctly through the public API.
2. Color accessor contract: After set_theme(name), every public color accessor (background_color(), input_text(), input_bg(), bold_color(), markdown_text_color()) returns the value from the theme that was set, not a stale default.
3. Custom theme contract: A custom theme loaded from a TOML file via load_theme(name, dir) produces a Theme whose color() returns the exact RGB values parsed from the file — the parse → store → read pipeline is lossless.
4. ThemeColor key coverage: Every variant of the ThemeColor enum is parseable via parse_theme_color() and resolvable via Theme::color() — no orphaned keys.

The branch now has 37 unit tests total (including the contract tests above). Would be great to get a review from @1jehuang when you have a chance.