Cost dashboard: TTS providers endpoint, header link, polish

Adds /api/costs/tts endpoint backed by get_tts_providers() in cost_db,
surfaces a Costs link in the control panel header, and small polish
to the costs page HTML/JS.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/index.html

@@ -17,6 +17,7 @@
                 <button id="export-session-btn">Export</button>
                 <button id="preflight-btn" class="preflight-btn">Preflight</button>
                 <button id="settings-btn">Settings</button>
+                <a href="/costs" class="header-link-btn" title="Cost Dashboard">Costs</a>
             </div>
             <div class="theme-bar">
                 <label for="show-theme-input" class="theme-label">Theme:</label>