feat: add --volume flag for audio gain control

[felix021]

Summary

• Add --volume multiplier (0.0–5.0) across CLI, MCP, and daemon
• Fix WAV spec mismatch in apply_wav_gain (always write 16-bit int)
• Fix SpeakOptions::default() volume from 0.0 to 1.0
• Validate volume range at all entry points (CLI rejects, MCP/daemon clamp)
• Update all 6 README translations with volume example

Test plan

• cargo build passes
• --volume 6.0 rejected by CLI
• GPU test: 1.0x vs 2.5x gain verified (exact 2.5x scaling, no clipping)
• WAV output format correct (Int 16bit) after gain

🤖 Generated with Claude Code

[pszymkowiak]

wshm · Automated triage by AI

📊 Automated PR Analysis

✨ Type	feature
🟡 Risk	medium

Summary

Adds a --volume flag (0.0–5.0) for audio gain control across CLI, MCP, and daemon entry points. Fixes WAV spec handling in apply_wav_gain to always output 16-bit int, corrects SpeakOptions default volume from 0.0 to 1.0, and updates all 6 README translations with volume examples.

Review Checklist

• Tests present
• Breaking change
• Docs updated

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Analyzed automatically by wshm · This is an automated analysis, not a human review.

[pszymkowiak]

Thanks for the clean implementation @felix021!

I rebased on main (resolved merge conflicts with the kokoro backend rewrite), and added a Volume selector to the TUI (vox setup) with presets from 0.5x to 3.0x -- wired to your --volume flag.

Changes I made during rebase:

• Resolved conflict in kokoro.rs (backend was rewritten from Python subprocess to native Rust)
• Added Volume screen to TUI with presets (0.5, 0.75, 1.0, 1.25, 1.5, 2.0, 3.0)
• Integrated selected_volume() into test_speak() so TUI test respects volume

LGTM, merging.