rebrand: rename VoiceBox to TalkBox throughout codebase

- All 'voicebox'/'Voicebox'/'VOICEBOX' strings replaced with 'talkbox'/'TalkBox'/'TALKBOX'
- Port changed from 17493 to 17494 (avoids conflict with upstream VoiceBox)
- MCP tool namespace: voicebox.* -> talkbox.*
- App bundle ID: sh.voicebox.app -> com.talkbox.app
- Binary names: voicebox-server -> talkbox-server, voicebox-mcp -> talkbox-mcp
- Docker user/group: voicebox -> talkbox
- Database: voicebox.db -> talkbox.db
- Env vars: VOICEBOX_* -> TALKBOX_*
- Asset files renamed: voicebox-logo.* -> talkbox-logo.*, etc.
- External binaries in tauri.conf.json updated to talkbox-server/talkbox-mcp

backend/main.py

@@ -1,4 +1,4 @@
-"""Entry point for the voicebox backend.
+"""Entry point for the talkbox backend.
 
 Imports the configured FastAPI app and provides a ``python -m backend.main``
 entry point for development.
@@ -11,7 +11,7 @@ from .app import app  # noqa: F401 -- re-export for uvicorn "backend.main:app"
 from . import config, database
 
 if __name__ == "__main__":
-    parser = argparse.ArgumentParser(description="voicebox backend server")
+    parser = argparse.ArgumentParser(description="talkbox backend server")
     parser.add_argument(
         "--host",
         type=str,