feat: multi-notebook support with colors, export/import, and auth-gated features

[onlyonehas]

Summary

Adds full multi-notebook support to Instant-Calc, letting users create, rename, delete, and organize multiple calculation notebooks with visual color labels. Signed-out users get a single notebook with tooltips prompting sign-in for advanced features.

Changes

• Tab bar – add/rename/delete/switch notebooks via toolbar tabs
• Notebook colors – each tab has a colored dot; click to open an inline 8-color palette
• Export/import – download all notebooks as JSON; upload to restore
• Database persistence – all notebooks + active notebook ID are saved/loaded via Firebase; backward-compatible with old single-notebook schema
• Auth gating – add/rename/delete/color/export/import are disabled for signed-out users with "Sign in for more features" tooltips
• Hydration fix – deduction dates moved from useState initializer to useEffect to prevent SSR mismatch
• Editor sync fix – setTimeout(0) in sign-out handler forces Quill editor sync after state updates settle

Testing

• npm run build – passes
• npm test – 41/41 tests pass
• npm run lint – no errors

[github-actions]

Visit the preview URL for this PR (updated for commit a5f5519):

https://instant-calc--pr55-feat-multi-notebooks-nk6v3dc2.web.app

_{(expires Mon, 03 Aug 2026 23:14:24 GMT)}

_{🔥 via Firebase Hosting GitHub Action 🌎}

_{Sign: 6bc8ccb070bbb04b63e3a5957eef152c83a6b443}