Add custom order notes

[SmallNeon]

Added note feature. Requesting merge.

[spinsirr]

Thanks for this @SmallNeon! I've opened #17 which picks up your work rebased on the current main, with two adjustments:

• Debounced the note save (400 ms + flush on blur) so a burst of typing coalesces into a single chrome.storage write and a single sync-queue entry instead of per-keystroke writes. The internal writeQueueRef serializer is no longer needed.
• Dropped the actions/checkout@v4 → v5 bumps — happy to land those in a follow-up PR if lint is actually failing on v4.

Credit is preserved via a Co-authored-by trailer on the commit. Leaving this PR open for you to close when it's convenient.