feat: add voice customization settings UI (#1017)

- Add /voice/settings page with sliders for speed (WPM) and volume,
  voice model selector, preview text input, and preview/save actions
- Add /voice/settings/data (GET) and /voice/settings/save (POST) endpoints
  to voice.py; settings persisted to data/voice_settings.json and applied
  live to the pyttsx3 TTS engine with graceful degradation
- Add VS* CSS classes to mission-control.css (range slider, select, actions)
- Link "VOICE SETTINGS" in desktop dropdown and mobile nav in base.html
- Preview uses browser Web Speech API (pitch/rate/volume) for instant feedback

Fixes #1017

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

static/css/mission-control.css

@@ -2547,3 +2547,120 @@
 .tower-adv-title { font-size: 0.85rem; font-weight: 600; color: var(--text-bright); }
 .tower-adv-detail { font-size: 0.8rem; color: var(--text); margin-top: 2px; }
 .tower-adv-action { font-size: 0.75rem; color: var(--green); margin-top: 4px; font-style: italic; }
+
+
+/* ── Voice settings ───────────────────────────────────────── */
+.voice-settings-page { max-width: 600px; margin: 0 auto; }
+
+.vs-field { margin-bottom: 1.5rem; }
+
+.vs-label {
+  display: block;
+  font-size: 0.75rem;
+  font-weight: 700;
+  letter-spacing: 0.1em;
+  color: var(--text-dim);
+  margin-bottom: 0.5rem;
+}
+.vs-value { color: var(--green); font-family: var(--font); }
+
+.vs-slider {
+  width: 100%;
+  -webkit-appearance: none;
+  appearance: none;
+  height: 4px;
+  background: var(--border);
+  border-radius: 2px;
+  outline: none;
+  cursor: pointer;
+}
+.vs-slider::-webkit-slider-thumb {
+  -webkit-appearance: none;
+  appearance: none;
+  width: 18px;
+  height: 18px;
+  border-radius: 50%;
+  background: var(--purple);
+  cursor: pointer;
+  box-shadow: 0 0 6px rgba(124, 58, 237, 0.5);
+  transition: box-shadow 0.2s;
+}
+.vs-slider::-webkit-slider-thumb:hover { box-shadow: 0 0 12px rgba(124, 58, 237, 0.8); }
+.vs-slider::-moz-range-thumb {
+  width: 18px;
+  height: 18px;
+  border-radius: 50%;
+  background: var(--purple);
+  cursor: pointer;
+  border: none;
+  box-shadow: 0 0 6px rgba(124, 58, 237, 0.5);
+}
+.vs-range-labels {
+  display: flex;
+  justify-content: space-between;
+  font-size: 0.7rem;
+  color: var(--text-dim);
+  margin-top: 0.25rem;
+}
+
+.vs-select,
+.vs-input {
+  width: 100%;
+  padding: 0.5rem 0.75rem;
+  background: var(--bg-card);
+  border: 1px solid var(--border);
+  border-radius: var(--radius-sm);
+  color: var(--text);
+  font-family: var(--font);
+  font-size: 0.9rem;
+}
+.vs-select { cursor: pointer; }
+.vs-select:focus,
+.vs-input:focus {
+  outline: none;
+  border-color: var(--purple);
+  box-shadow: 0 0 0 2px rgba(124, 58, 237, 0.2);
+}
+
+.vs-unavailable {
+  font-size: 0.85rem;
+  color: var(--text-dim);
+  padding: 0.5rem 0.75rem;
+  border: 1px dashed var(--border);
+  border-radius: var(--radius-sm);
+}
+
+.vs-actions {
+  display: flex;
+  gap: 0.75rem;
+  margin-top: 1.5rem;
+  flex-wrap: wrap;
+}
+.vs-btn-preview,
+.vs-btn-save {
+  flex: 1;
+  padding: 0.6rem 1.2rem;
+  border-radius: var(--radius-sm);
+  font-family: var(--font);
+  font-size: 0.85rem;
+  font-weight: 700;
+  letter-spacing: 0.08em;
+  cursor: pointer;
+  min-height: 44px;
+  transition: opacity 0.2s, box-shadow 0.2s, background 0.2s;
+}
+.vs-btn-preview {
+  background: transparent;
+  border: 1px solid var(--purple);
+  color: var(--purple);
+}
+.vs-btn-preview:hover {
+  background: rgba(124, 58, 237, 0.15);
+  box-shadow: 0 0 8px rgba(124, 58, 237, 0.3);
+}
+.vs-btn-save {
+  background: var(--green);
+  border: none;
+  color: var(--bg-deep);
+}
+.vs-btn-save:hover { opacity: 0.85; }