TUI Audio/VAD float serialization (#451)

The TUI used to write volume = "0.70" and threshold = "0.50" as quoted strings, which the daemon's serde config rejected with invalid type: string "0.70", expected f32. Saving any Audio or VAD change broke the daemon on next start. Now serialized as bare TOML floats, with legacy string and int forms still tolerated on load so existing configs migrate on first save.

Unit tests

cargo test --lib tui::config_editor::tests::set_f32 -- --nocapture
cargo test --lib tui::config_editor::tests::get_f32_or -- --nocapture
# Expected: set_f32_writes_toml_number_not_string passes
# Expected: get_f32_or_recovers_legacy_string_and_int_forms passes

Structural verification

grep -c "set_string.*format!" src/tui/audio.rs src/tui/vad_section.rs
# Expected: 0 (the anti-pattern is gone)

grep -c "set_f32\|get_f32_or" src/tui/audio.rs src/tui/vad_section.rs
# Expected: 4+ (each section reads + writes)

Runtime test

1. Open the TUI: voxtype configure
2. Audio section, change Volume, Save
3. Inspect ~/.config/voxtype/config.toml:

   grep -A1 "audio.feedback" ~/.config/voxtype/config.toml
   # Expected: volume = 0.7 (bare number, no quotes)

4. systemctl --user restart voxtype Expected: daemon starts cleanly, no f32 parse error in journalctl

Migration test

Hand-edit a quoted volume back in, then save via the TUI:

sed -i 's/^volume = 0\./volume = "0./; s/^\(volume = "0\.[0-9]*\)$/\1"/' ~/.config/voxtype/config.toml

Open the TUI, save, re-check:

grep volume ~/.config/voxtype/config.toml
# Expected: bare TOML number again after the save (auto-migration worked)