Add persistent default color setting for /color command

Feature Request

Add a persistent defaultColor setting (in ~/.claude.json or via /config) so the /color command preference carries across sessions.

Current Behavior

/color yellow sets the input bar color for the current session only. Every new session resets to the default. Users who prefer a specific color must manually run /color at the start of every session.

Desired Behavior

A persistent setting like:

// ~/.claude.json
{
  "preferences": {
    "defaultColor": "yellow"
  }
}

Or configurable via /config → Appearance → Default Color.

Use Case

Power users who run many concurrent sessions (e.g., via tmux with project-specific shortcuts) want visual consistency without manual setup each time. A default color setting would also help distinguish Claude Code sessions from regular terminal windows at a glance.

Alternatives Considered

• Wrapping the claude command to pipe /color on startup — doesn't work cleanly with the interactive TUI
• SessionStart hooks — can't send commands to the Claude input stream
• Manual /color each session — works but repetitive