TTS: settings UI — provider/voice/speed picker + API key entry

[thefourcraft]

Part of the Read Aloud (TTS) epic.

Scope

• New TextToSpeechSettingsView (Settings tab + ContentView nav entry).
• Enable toggle; provider picker; per-provider voice dropdown; speed/rate slider; 'auto-restore selection' behavior.
• API key entry + 'Verify' reusing APIKeyManagementView patterns for inworld/cartesia (others reuse existing key fields).
• Local model card for Kokoro download/delete.
• Hotkey recorder row for the read-aloud shortcut.

Files: Views/Settings/TextToSpeechSettingsView.swift, nav wiring in ContentView.swift.