feat: complete Phase 12 — voice playback ticker, cache, config, and UI

Add playback position ticker in event loop with 1s UI refresh rate,
integrate VoiceCache for downloaded voice files, add [audio] config
section (cache_size_mb, auto_download_voice), and render progress bar
with waveform visualization in message bubbles.

Fix race conditions in AudioPlayer: add `starting` flag to prevent
false `is_stopped()` during ffplay startup, guard pid cleanup so old
threads don't overwrite newer process pids. Implement `resume_from()`
with ffplay `-ss` for real audio seek on unpause (-1s rewind).

Kill ffplay on app exit via `stop_playback()` in shutdown + Drop impl.

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

Cargo.toml

@@ -36,6 +36,7 @@ dirs = "5.0"
 thiserror = "1.0"
 tracing = "0.1"
 tracing-subscriber = { version = "0.3", features = ["env-filter"] }
+base64 = "0.22.1"
 
 [dev-dependencies]
 insta = "1.34"