fix(desktop): open settings by default

[elijah-potter]

Issues

N/A

Description

Open Harper Desktop settings automatically on first launch only.

• Added a persisted-config existence check in Config::main_config_exists().
• Updated desktop startup to treat a missing config file as first launch.
• Save the default config during first launch so subsequent launches do not reopen settings automatically.
• Open the settings window from Tauri setup only when first-launch detection succeeds.

The goal is to make it more obvious that the settings must be accessed to go through the setup process, and thereby give Harper accessibility permissions.

Demo

N/A

How Has This Been Tested?

• Added unit coverage for the config-existence helper.

AI Disclosure

• I am a human and didn't use any AI.
• I used LLM features of my editor, but not an agent.
• I used an AI agent interactively.
• I am an agent or I got an agent to do the work autonomously.

If Your PR Implements or Enhances a Linter

N/A

Checklist

• I have performed a self-review of my own code
• I have added tests to cover my changes
• I have considered splitting this into smaller pull requests.