feat: per-theme descriptions + accent colour in /theme completion

[subzeroid]

Bug reported

In the /theme <name> completion dropdown, every theme showed the same meta line ("theme to switch to; omit to show the active theme + list") — it was the argparse help for the name arg, reused for every choice. Users also expected the banner to live-preview while arrowing the dropdown.

What changed

• Per-theme descriptions. New theme_description(name) in ui/theme.py; the completion dropdown now shows each theme's own line:

  aiograpi   | instagrapi violet → blue (default)
  amber      | 80s amber CRT phosphor
  claude     | Claude Code burnt orange
  cyberpunk  | neon cyan → magenta → green
  hacker     | matrix phosphor green on black
  instagram  | Instagram brand gradient
  

• Accent colour. Each theme name in the popup renders in its own accent colour (_theme_label) — a colour hint before you commit. The picker footer shows the description too.

On live preview

Completion menus in prompt_toolkit are passive — there's no "on-highlighted" callback, so the banner can't repaint as you arrow the dropdown. Full live banner preview lives in the bare /theme picker (/theme + Enter → full-screen, ↑/↓ repaints the banner). The dropdown's colour + description is the most a passive menu can offer. (User chose "both": picker for preview + colour/descriptions in the dropdown.)

Tests

• theme_description exists for every theme, all distinct; hacker mentions "green".
• /theme arg completions carry per-theme meta (different across themes).
• Full gate green: ruff / format / mypy / pytest --cov=insto --cov-fail-under=80 → 899 passed, coverage 81.89%.

🤖 Generated with Claude Code