feat: live AzuraCast radio mode for footer player (#65)

[adurrr]

Adds a new <podcast-live> Web Component that polls an AzuraCast nowplaying API and renders real-time track metadata in the persistent footer.

What it does

• New <podcast-live> element placed as a sibling of <podcast-footer> in baseof.html. Polls a configurable AzuraCast nowplaying API at a default 15s interval (60s when idle, exponential backoff on errors).
• Renders a red LIVE badge with a pulse animation (respects prefers-reduced-motion: reduce), the current track title, artist, and a 24H start→end time computed from the track's elapsed and duration.
• A pulsing LIVE badge + the badge's aria-label ("Live radio currently broadcasting") signals the live state.
• A Listen Live button in the regular footer appears when a <podcast-live> is configured and the current audio source is NOT the live stream. Clicking it dispatches podcast-play with the live URL → single-stream enforcement automatically pauses any other source.
• No autoplay — the user must click Listen Live (or the PodcastLive's own button) to start.
• URL sanitization: data-azuracast-api-url MUST be HTTPS (enforced at both template-time and runtime). javascript:, data:, malformed URLs all set state to "error" and skip fetch.
• Configuration persists across htmx / Turbo / Turbolinks navigation via the standard persistence markers.

Configuration

{{</* podcast-player
  src=
  source=azuracast
  data-azuracast-api-url="https://radio.example.org/api/live/nowplaying/station-slug"
  live-mode="true"
*/>}}

The example site demonstrates with Quantum Radio (radio.erb.pw/api/live/nowplaying/subspace). See exampleSite/content/posts/live-mode.md for a working example.

Latent bug fix

The existing AzuracastAdapter was reading azuracast-api-url (no data- prefix) but the docs example used data-azuracast-api-url. Users following the docs got "missing azuracast-api-url attribute" errors. Standardized on the documented data-azuracast-api-url with backward-compat fallback to the old name.

Test plan (TDD)

• npx vitest run — 166/166 tests pass (32 new + 134 pre-existing)
  • 32 new <podcast-live> tests covering: registration/rendering, state machine, polling, single-stream integration, 24H time formatting, URL sanitization, CSS animation, cleanup
  • Test corrections: replaced vi.runAllTimersAsync (fires future timers) with vi.runOnlyPendingTimersAsync where appropriate, then with await Promise.resolve() chains for pure microtask draining; fixed off-by-one in C4/C5 backoff timing tests
• go test -v -timeout 120s ./tests/hugo/... — 3 new shortcode tests pass (positive / omitted / bad-scheme); full suite still passes
• npm run format:check — passes
• No em-dashes in new doc or CHANGELOG content

Files changed

File	Change
assets/js/podcast-player.js	New PodcastLive class (250+ lines); AzuracastAdapter attribute fix; "Listen Live" button + handler in footer; new i18n keys in both classes' DEFAULT_I18N
layouts/_shortcodes/podcast-player.html	New live-mode and data-azuracast-api-url params (HTTPS validation)
i18n/en.toml + exampleSite/i18n/es.toml	New player_listen_live key
exampleSite/layouts/_default/baseof.html	<podcast-live> element (Quantum Radio) + i18nKeys slice update
exampleSite/content/posts/live-mode.md	New demo post
exampleSite/content/docs/advanced.md + homepage-setup.md + ES	New "Live Mode" / "Live Radio Mode" sections
tests/js/web-component.test.js	32 new Vitest tests + corrections to existing tests
tests/hugo/shortcode_test.go + 2 new testdata/ dirs	3 new Go tests

What's NOT in this PR

• E2E test for the live flow — added to the plan but deferred. Manual testing on localhost will exercise the full flow (cd exampleSite && hugo server → visit /posts/live-mode/ → click Listen Live → see Quantum Radio's now-playing). The unit tests cover the component logic; E2E can be added in a follow-up.
• The issue mentions <podcast-live> could be placed INSIDE <podcast-footer> (via slot). I chose to place it as a SIBLING because the footer has no <slot> and slot-based composition would require coordinating two lifecycles. The sibling approach is simpler and visually equivalent (CSS positions the live card next to the footer bar).

Closes #65