feat: comprehensive visual redesign inspired by LibreChat/Claude.ai

- New design tokens: Inter font, indigo primary, purple dark mode accent,
  full radius/shadow/transition token set, 260px sidebar
- Welcome screen with agent cards (Claude Code, OpenCode, Gemini, Kilo, Codex)
  replaces blank canvas when no conversation is active
- Input area redesigned as rounded card with textarea + toolbar (agent/model
  chips + action buttons), matching ChatGPT/Claude.ai style
- Sidebar header: logo + ··· overflow menu replaces 5 cramped buttons;
  Import/Archived/Clone/Clear All moved into dropdown
- Streaming status bar above input shows agent name + typing dots + cancel
- Date-grouped conversation list (Today/Yesterday/Last 7 days/Older) with
  relative timestamps and per-agent color dots
- Typing dots animation for streaming indicator
- Message bubble CSS (user right-aligned accent, agent left-aligned flat)
- Modal backdrop-filter blur + scale-in animation
- Thin hover-reveal scrollbars

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: AgentGUI