Landing page: testimonials slider, how-it-works page, 25 TTS voices

- Add testimonial slider with 8 fake caller reviews
- Add how-it-works page with visual architecture diagram
- Expand voice pools: Inworld 25 voices (14M/11F), ElevenLabs 22 (14M/8F)
- Voice pools auto-switch when TTS provider changes
- Add cover art locally, update cache-busted image refs
- Add "More from Luke" footer links (MMG, prints, YouTube)
- Ad channel configurable in settings UI

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

docs/architecture.md

@@ -35,7 +35,7 @@ Session Reset / First Access to Caller Slot
     │
     ▼
 _randomize_callers()
-    │  Assigns unique names (from 24M/24F pool) and voices (5M/5F) to 10 slots
+    │  Assigns unique names (from 24M/24F pool) and voices (Inworld: 14M/11F, ElevenLabs: 14M/8F) to 10 slots
     │
     ▼
 generate_caller_background(base)