feat: configurable semantic role colors in themes and config

[wowi42]

Summary

Implements item 3 of the "Input and pickers" section of #186: "Extend the theme/config system to map semantic roles (agent, error, tool, permission) to configurable colors."

Semantic conversation roles are now recolorable from both theme files and the [colors] config section, via a new roles map:

{
  "colors": {
    "chat_background": "#1e1e2e",
    "roles": {
      "agent": "#cdd6f4",
      "error": "#f38ba8",
      "tool": "#f9e2af",
      "permission": "#cba6f7"
    }
  }
}

Changes

• New src/ui/roles.rs: global registry of role color overrides with the historical palette as defaults. BlockStyle::color() resolves through it, so all feed blocks — and legacy write_line(color) call sites that map to roles via style_from_color — pick up themed colors automatically.
• All 10 roles supported: user, agent, reasoning, tool, tool_result, error, system, welcome, permission, plain. Colors are parsed with the existing parse_color (named colors + #rrggbb).
• Robust application: unknown role names and unparsable colors are skipped with a tracing warning, never breaking rendering. Applying a theme or config without a roles map resets to the default palette, so switching themes doesn't leak the previous theme's roles.
• Docs: docs/CONFIG.md colors section documents roles with an example.
• No changes to bundled themes; default behavior is pixel-identical to before.

Verification

• cargo test (717 passed), cargo test --no-default-features (602 passed), cargo clippy --all-features -- -D warnings and cargo clippy --no-default-features -- -D warnings — all clean.
• New unit tests for the registry (override/fallback/reset, role-name parsing, bad input handling).
• Smoke-tested on a pty with a roles theme: themed colors render in the actual TUI output.

Refs #186