Persist pencil color

[OhPointFive]

This PR automatically saves the user's pencil color, so it will no longer reset to gray on page reload or when starting a new puzzle.

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Name	Status	Preview	Comments	Updated (UTC)
downforacross.com	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Dec 16, 2023 2:46am

[sweep-ai[bot]]

Apply Sweep Rules to your PR?

• [ ] Apply: All new business logic should have corresponding unit tests.
• [ ] Apply: Refactor large functions to be more modular.
• [ ] Apply: Add docstrings to all functions and file headers.

[stevenhao]

looks good!