Add daily theme welcome nudge

[Jesssullivan]

Summary

• Add a desktop-only daily Skeleton Popover nudge to the existing theme switcher.
• Persist the nudge cadence with localStorage and keep manual theme interactions suppressing it for the day.
• Add Playwright regression coverage for daily cadence, desktop-only behavior, Skeleton/Zag popover anatomy, and dark-mode style cascade inheritance.

Verification

• Verified against Skeleton v4.15.2 / v5-next package metadata: stable is 4.15.2, next is 5.0.0-next.8, both depend on Zag popover/tooltip.
• Confirmed Skeleton Popover is the right primitive for interactive links; direct Floating UI is only an advanced/custom attachment path and does not provide Popover accessibility out of the box.
• npm run check
• npm run lint
• npx prettier --check src/lib/components/ThemeSwitcher.svelte e2e/dark-mode.spec.ts
• git diff --cached --check

Shadow CI

• Build shadow image: https://github.com/Jesssullivan/jesssullivan.github.io/actions/runs/25648826975
• Publish shadow preview: https://github.com/Jesssullivan/jesssullivan.github.io/actions/runs/25648840886
• Shadow image tag: shadow-shadow-deploy-theme-nudge-a7833a17ab2c-amd64