feat: adopt Tailwind CSS utility classes for styling

[nicomiguelino]

Summary

• Reworks index.html to use Tailwind CSS utility classes (with arbitrary-value bracket syntax for exact px/rem/gradients/shadows/colors) instead of custom app-owned CSS classes. Visual appearance and behavior are unchanged — this is a pure refactor.
• Slims the CSS entry point (src/css/style.css) down to just the @screenly/edge-apps/styles design-system import layered correctly against Tailwind's theme/utilities layers. Deletes src/css/grid.css and src/css/kanban.css, since all their rules are now expressed as Tailwind utility classes directly on the corresponding elements (both static markup in index.html and dynamically-created elements in src/app.ts / src/kanban.ts).
• Class names referenced by tests/JS (kanban-card, kanban-card-title, kanban-column, kanban-column-header, kanban-column-dot, kanban-column-title, kanban-cards, cell-pills, pill) are preserved verbatim alongside the new utility classes.
• Table header/body rows and cells, which were previously styled purely via element/tag selectors (#data-table th, #data-table td, etc.) with no className set in src/app.ts, now get Tailwind utility classNames assigned at creation time. The "divider" line between rows (all but the last row) is now expressed via the [&:not(:last-child)]:bg-[...] arbitrary-variant pattern instead of a static CSS :last-child rule — same visual result, recomputed automatically by the browser as rows are added/removed by trimRowsToFit().
• Bumps @screenly/edge-apps to ^1.3.0 (unpublished; ships Tailwind support).
• Regenerates screenshots/* (grid + kanban views, all resolutions) to reflect the change (pixel-identical to before).

Reference/pilot migrations for this same rollout:

• 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 lint — passes
• bun test --pass-with-no-tests src/ — 51 tests pass (including class-name assertions in kanban.test.ts / render.test.ts)
• bun run format:check — passes
• bun run build — succeeds
• bun run screenshots — all 20 Playwright screenshot tests pass; visually confirmed grid view (landscape + portrait) and kanban view render correctly with no missing text or broken layout
• Verified package.json/bun.lock are clean of the temporary local-tarball reference used to test against the unpublished @screenly/edge-apps@1.3.0 build; only the version bump string is committed, lockfile untouched