Add built-in color presets with one-command switching #1243

Problem

Jcode exposes all 22 semantic color roles through /colors, but switching to a cohesive theme currently requires manually setting every role or pasting TOML into config.toml. This makes experimentation and sharing themes cumbersome.

Proposed behavior

• Add a small set of accessible built-in color presets.
• Let users list presets and apply one with a single local command.
• Persist the selected palette through the existing [display.colors] configuration.
• Apply changes immediately without restarting.
• Keep /theme dark|light|auto focused on terminal background mode.

Example:

/colors presets
/colors preset sonic

Acceptance criteria

• Presets populate all semantic roles.
• Unknown names return a useful error with available choices.
• Existing custom colors and reset behavior remain unchanged.
• Command completion, help text, documentation, and tests cover the new workflow.