TASK-033 TUI: Auto-select dark/light theme based on macOS appearance

Description

On startup, detect macOS Dark Mode and choose the matching theme variant automatically, rather than always defaulting to flexoki.

Detection:

import subprocess
result = subprocess.run(
    ["defaults", "read", "-g", "AppleInterfaceStyle"],
    capture_output=True, text=True
)
is_dark = result.returncode == 0 and "Dark" in result.stdout

Startup behavior:

• If state file (TASK-021) has a saved theme: use it as-is (user explicitly chose it)
• If no saved theme: pick flexoki-dark when Dark Mode is on, flexoki when off
• t cycling still works normally and saves to state file
• A --no-auto-theme CLI flag or constant at top of file disables this behavior

Acceptance Criteria

• #1 First launch with no saved theme picks dark variant in Dark Mode, light variant otherwise
• #2 Saved theme from state file takes priority over auto-detection
• #3 Detection failure (non-macOS, permission error) falls back to flexoki silently
• #4 Auto-detection runs synchronously before the app loop starts (no flash of wrong theme)