fix(tts): progressive Persian playback for real browser content

[AmirMotefaker]

Fixes #69.

Real-browser blocker

The merged direct-fetch fix correctly prevents an infinite hang, but realistic Chrome content still reaches the 90s startup timeout, including Summary mode.

Root cause

The Worker may synthesize multiple narration chunks before returning a single WAV. That means browser playback waits for the whole narration instead of starting from the first available segment.

Fix

• split prepared Persian narration into ~900-character playback chunks in the extension;
• synthesize only the current chunk;
• start playback immediately when the first chunk arrives;
• automatically request the next chunk after the current audio ends;
• show visible progress (بخش x از y);
• preserve female/male voice selection and Play/Pause/Resume/Stop;
• on later server failure, browser fallback continues from remaining chunks instead of restarting the whole article;
• retain the bounded 90s timeout per chunk;
• no secret exposure and no stable/store publication.

Acceptance

CI + candidate + Cloudflare E2E must pass, then repeat the exact real Chrome summary/full tests before Stable.

Refs #61 #65 #67 #68 #69