Feature: /theme slash command and custom theme palettes

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.

Start by reviewing the feature/themes-light-dark-config branch and the jcode-config-types, jcode-tui-style, and jcode-tui changes. Run the theme and tui-style tests, then verify the /theme command, built-in and custom palettes, TOML theme loading, background filling, and terminal-adaptation behavior described in the issue.