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/styles.css

@@ -761,6 +761,97 @@ body {
   color: #22c55e;
 }
 
+/* Token Counter */
+.token-counter {
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  font-size: 12px;
+  color: var(--text-secondary);
+}
+
+.token-count {
+  padding: 4px 8px;
+  background: var(--bg-tertiary);
+  border: 1px solid var(--border);
+  border-radius: 6px;
+}
+
+.token-details-btn {
+  width: 20px;
+  height: 20px;
+  border: none;
+  background: transparent;
+  color: var(--text-secondary);
+  border-radius: 4px;
+  cursor: pointer;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  transition: all 0.2s ease;
+  padding: 0;
+}
+
+.token-details-btn:hover {
+  background: var(--bg-tertiary);
+  color: var(--accent);
+}
+
+.token-breakdown {
+  position: absolute;
+  bottom: 52px;
+  right: 16px;
+  background: var(--bg-secondary);
+  border: 1px solid var(--border);
+  border-radius: 8px;
+  padding: 12px;
+  box-shadow: 0 4px 12px rgba(0, 0, 0, 0.3);
+  z-index: 100;
+  min-width: 250px;
+}
+
+.token-breakdown-header {
+  font-size: 13px;
+  font-weight: 600;
+  color: var(--text-primary);
+  margin-bottom: 8px;
+  padding-bottom: 8px;
+  border-bottom: 1px solid var(--border);
+}
+
+.token-breakdown-list {
+  display: flex;
+  flex-direction: column;
+  gap: 6px;
+}
+
+.token-breakdown-item {
+  display: flex;
+  justify-content: space-between;
+  font-size: 12px;
+  color: var(--text-secondary);
+}
+
+.token-breakdown-item span:first-child {
+  color: var(--text-secondary);
+}
+
+.token-breakdown-item span:last-child {
+  color: var(--text-primary);
+  font-weight: 500;
+}
+
+.token-breakdown-total {
+  display: flex;
+  justify-content: space-between;
+  font-size: 13px;
+  font-weight: 600;
+  color: var(--accent);
+  margin-top: 8px;
+  padding-top: 8px;
+  border-top: 1px solid var(--border);
+}
+
 @keyframes pulse {
   0%, 100% {
     opacity: 1;