feat: add profile set command to update default voice/lang

[ppikrorngarn]

Summary

Adds a new profile set subcommand that lets users update the default voice and/or language of an existing profile without having to delete and recreate it.

Usage

# Set default voice
ttscli profile set gcp:default --voice en-US-Chirp3-HD-Kore

# Set both voice and lang
ttscli profile set gcp:default --voice en-US-Chirp3-HD-Kore --lang en-US

# Short flags
ttscli profile set gcp:default -v en-US-Chirp3-HD-Kore -l en-US

Changes

• internal/cli/cli.go — added ProfileSet constant, DefaultLang field on Config, and parser for profile set with --voice/--lang flags (at least one required)
• internal/app/command_profile.go — added runProfileSet handler that loads config, updates profile.Defaults, saves, and prints confirmation

Testing

All existing tests pass (go test ./...).