feat: adopt Tailwind CSS utility classes for styling

[nicomiguelino]

Summary

Migrates this repo to Tailwind CSS utility classes as part of the company-wide rollout across Edge App repos. This is a pure styling refactor — visual appearance and behavior are unchanged.

• index.html: replaced custom CSS classes with Tailwind utility classes (arbitrary-value bracket syntax for exact px/rem/color values, portrait: variant for @media (orientation: portrait) overrides). Original class names (header, error-card, channel-card, message, etc.) are kept alongside the new utility classes for stability/readability even though nothing currently depends on them in JS/tests.
• src/render/index.ts: dynamically created elements (channel cards, message rows, empty state) now get their Tailwind utility classes appended to className alongside the original class name — no change to rendering logic.
• src/css/style.css: slimmed down to just the Tailwind layer + design-system import boilerplate (no app-specific custom CSS remains, since this app has no @font-face or other runtime-driven styling).
• package.json: bumped @screenly/edge-apps to ^1.3.0 (unpublished; ships Tailwind support). Lockfile untouched — only used a local tarball temporarily to build/test, then reverted.

Reference PRs used as the pattern for this migration:

• feat: adopt Tailwind CSS utility classes for styling clock-app#12
• feat: adopt Tailwind CSS utility classes for styling weather-app#6
• feat: adopt Tailwind CSS utility classes for styling rss-reader-app#31 (for the dynamic-list rendering pattern)

Notes / deviations

• development branch existed and was already in sync with main, so this PR targets development per the standard workflow.
• Unlike the reference apps, this app has no background image / text-shadow, so body only needed bg-[#111] text-[#f2f2f3].
• CSS custom properties (--color-surface, --card-radius, etc.) were inlined as literal Tailwind arbitrary values, consistent with the reference PRs.
• Added explicit m-0 on <h2>/<p> in the error card, since the design system's reset only zeroes margin/padding on html/body, not on all elements (the app's own removed * { margin: 0 } rule previously handled this).
• Preserved word-break: break-word behavior (not Tailwind's break-words, which maps to overflow-wrap) via the arbitrary property [word-break:break-word] on message/error text.

Test plan

• bun run type-check — passes
• bun run build (production) — builds successfully, Tailwind utilities compiled into dist/css/style.css
• bun test — 10/10 tests pass
• bun run screenshots — 12/12 Playwright screenshot tests pass; regenerated screenshots reviewed visually (landscape feed, portrait feed, error screen) and match the original design