fix: Remove redundant audio validation to solve Issue #16 "Audio too quiet"

This commit fixes the false "Audio too quiet - please speak louder" errors
that users were experiencing even when speaking at normal volume.

Problem:
- AudioValidator was using crude energy/amplitude thresholds (0.01 RMS, 0.05 peak)
- These thresholds were rejecting normal speech as "too quiet"
- The validator was duplicating work already done by the transcriber
- WAV files were being read and validated twice unnecessarily

Solution:
- Completely removed AudioValidator module and all its validation logic
- Moved only the essential duration check (0.5s minimum) to transcriber
- Let Whisper's sophisticated VAD handle speech detection with proper parameters:
  - no_speech_thold: 0.6
  - entropy_thold: 2.4
  - logprob_thold: -1.0
- Single validation point, single file read, better performance

Impact:
- Users speaking at normal volume will no longer get false rejection errors
- Improved performance by eliminating duplicate file reads
- Cleaner architecture with proper separation of concerns
- Whisper AI properly determines if speech is present, not crude math

Testing:
- All 143 tests pass
- Recordings under 0.5s are still properly rejected as "too short"
- Silent recordings return "No speech detected" from Whisper (correct)

Fixes #16

Author: moinulmoin

src-tauri/src/audio/mod.rs

@@ -2,4 +2,3 @@ pub mod level_meter;
 pub mod recorder;
 pub mod resampler;
 pub mod silence_detector;
-pub mod validator;