feat: implement token counter with real-time breakdown

Add comprehensive token counting functionality to provide visibility into
context usage:

Backend (Rust):
- Add tiktoken-rs dependency for OpenAI-compatible token counting
- Implement get_token_count command with detailed breakdown
- Count tokens for: system prompt, preset instructions, persona, world info,
  author's note, message history, and current input
- Per-section token breakdown for optimization insights

Frontend (JavaScript/HTML/CSS):
- Add token counter widget in status bar
- Real-time updates as user types (debounced 300ms)
- Expandable breakdown tooltip showing per-section counts
- Automatic update when chat history loads or changes
- Clean, minimal UI with hover interactions

Features:
- Accurate token counting using cl100k_base tokenizer
- Debounced updates for performance
- Detailed breakdown by context section
- Visual indicator with total token count
- Click to expand/collapse detailed breakdown
- Auto-hide when no character is active

This completes the "Must-Have for Basic Roleplay" features from the roadmap:
✅ World Info/Lorebooks
✅ Author's Note
✅ Token Counter
- Message Examples Usage (next)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src-tauri/Cargo.toml

@@ -32,4 +32,6 @@ png = "0.17"
 base64 = "0.21"
 image = "0.24"
 regex = "1"
+chrono = "0.4"
+tiktoken-rs = "0.5"