feat: adopt Tailwind CSS utility classes for styling

[nicomiguelino]

Summary

• Rework index.html to use Tailwind utility classes (with arbitrary-value bracket syntax for exact px/rem/gradient/shadow values) instead of custom CSS classes, keeping visual appearance and behavior unchanged.
• Use the portrait: variant for the existing @media (orientation: portrait) overrides (timer card size, date/digit font sizes).
• Slim src/css/style.css down to the standard Tailwind + @screenly/edge-apps/styles layered import, keeping only the custom @font-face rules for Inria Sans (not part of the design system tokens) and the --theme-color-primary custom property (read by progress-ring.ts via getComputedStyle).
• No JS/TS changes needed — all interactive elements are targeted via data-* attributes and #id/svg[data-tick] selectors, none of which relied on the removed custom class names.
• Bump @screenly/edge-apps to ^1.3.0 (unpublished; adds Tailwind CSS support) in package.json.
• Regenerate all 10 screenshots to reflect the (visually identical) refactor.

This is part of the company-wide rollout of Tailwind CSS across edge-app repos. Reference/pilot implementations:

• feat: adopt Tailwind CSS utility classes for styling clock-app#12
• feat: adopt Tailwind CSS utility classes for styling weather-app#6

Test plan

• bun run type-check passes
• bun run build succeeds (only pre-existing/expected warnings, matching reference PRs)
• bun run screenshots — all 10 Playwright screenshot tests pass
• Visually reviewed landscape (1920x1080), portrait (1080x1920), and small-resolution (800x480) screenshots — timer card, progress ring, date text, and accent-colored timer digits all render correctly with no layout regressions
• Temporary local tarball install of the unpublished @screenly/edge-apps@1.3.0 build used only for local verification, then fully reverted (package.json + bun.lock restored to committed state, with only the version-string bump re-applied); lockfile is untouched in this PR