fix: improve Android reader scroll and system TTS

[codedogQBY]

Summary

• Throttle high-frequency reader relocate UI/context updates to reduce Android scroll-mode JS pressure.
• Remove noisy reader/TTS debug logs that were flooding diagnostics during scroll/TTS updates.
• Fall back to Expo Speech when the native system TTS + TrackPlayer startup path fails, matching the settings preview path more closely.

Fixes #547
Fixes #548
Fixes #549

Verification

• ./node_modules/.bin/tsc -p packages/app-expo/tsconfig.json --noEmit
• ./node_modules/.bin/biome check packages/app-expo/src/stores/tts-store.ts
• git diff --check