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/css/style.css

@@ -108,11 +108,24 @@ header button {
     transition: all 0.2s;
 }
 
-header button:hover {
+header button:hover, .header-link-btn:hover {
     background: #3a2e1f;
     border-color: rgba(232, 121, 29, 0.3);
 }
 
+.header-link-btn {
+    background: var(--bg-light);
+    color: var(--text);
+    border: 1px solid rgba(232, 121, 29, 0.15);
+    padding: 8px 16px;
+    border-radius: var(--radius-sm);
+    cursor: pointer;
+    transition: all 0.2s;
+    text-decoration: none;
+    font-size: inherit;
+    font-family: inherit;
+}
+
 .theme-bar {
     display: flex;
     align-items: center;