feat: adopt Tailwind CSS utility classes for styling

[nicomiguelino]

Summary

• Reworks index.html to use Tailwind CSS utility classes instead of custom CSS classes, using arbitrary-value bracket syntax (text-[...], bg-[linear-gradient(...)], shadow-[...], border-[...], rounded-[...]) to reproduce exact px/rem/gradient/shadow values from the original design.
• Preserves all class names referenced by src/main.ts (.feed-card-title, .feed-card-date, .feed-card-source, .feed-card-excerpt) alongside the new utility classes.
• Expresses the portrait-only .feed-card:nth-child(n+4) { display: none } rule as a self-referential arbitrary variant (portrait:[&:nth-child(n+4)]:hidden) directly on the templated card element, and reproduces the [hidden]-attribute override behavior for #feed-grid/#feed-error via [&[hidden]]:hidden, so no JS changes were needed.
• Slims src/css/style.css down to just the Tailwind layer setup (@layer theme, base, utilities + the three imports), dropping all app-specific custom CSS now expressed as utilities. No @font-face needed — Inter is already provided by @screenly/edge-apps's base styles.
• Bumps @screenly/edge-apps to ^1.3.0 (unreleased version that will ship Tailwind CSS support) in package.json. The lockfile intentionally stays as committed (pointing at 1.0.0) until 1.3.0 is actually published — this matches how the reference PRs handled it.
• Regenerates all screenshots/*.webp files.

This follows the same conversion pattern already applied to:

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

Note: this repo has no development branch (only main), so this PR targets main directly, per guidance from the migration coordinator.

Test plan

• bun run type-check passes
• bun run build succeeds (pre-existing unrelated Rolldown import.meta/chunk-size warnings from @screenly/edge-apps's own bundled dev-tools/locale code, not caused by this change)
• bun run screenshots (Playwright) regenerated successfully for all configured resolutions, tested against a local unpublished @screenly/edge-apps@1.3.0 tarball build (reverted afterwards; only the package.json version bump was kept, lockfile untouched)
• Visually verified landscape (1920x1080), portrait (1080x1920), and a small-format (800x480) screenshot — layout, card grid, text, and the portrait 3-card limit all render correctly and match the pre-conversion design pixel-for-pixel (verified by diffing against the previous committed 1920x1080 screenshot)