feat: add seeded squiggle background

[yckonnnnn]

Summary

Add a seeded Squiggle background generator with palette-colored strokes. Density sets row spacing, scale sets stroke width, and rotation controls the pattern angle. Closes #153.

Screenshots

The Squiggle option appears in Background Studio. I checked its preview and exported SVG and PNG files in the browser; the CSS export also renders as a data-URI background.

Checklist

• pnpm typecheck, pnpm test, pnpm lint, pnpm format:check, pnpm check:dashes and pnpm build pass
• Conventional commit message
• Checked in dark and light themes and at mobile width (375 px)
• Checked keyboard navigation with the End key
• No any, console.log, or new external services
• Updated docs and CHANGELOG.md

Breaking changes

None.

Testing completed

All required checks pass. The registry tests verify deterministic output, valid SVG, CSS export, and rotation. In the browser, I selected Squiggle, checked the light theme at 375 px, and downloaded SVG, 1x PNG, and 2x PNG exports.

[vercel]

@yckonnnnn is attempting to deploy a commit to the yashkewlani2020-gmailcom's projects Team on Vercel.

A member of the Team first needs to authorize it.

[github-actions]

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