feat: configurable footer player width via size attribute (#62)

[adurrr]

A new size attribute on <podcast-footer> controls the bar's visual footprint through CSS custom properties:

• size="small" (default): the current compact bar. 36px cover, 140px info column, 32px buttons.
• size="medium": a wider bar with a 48px cover, 240px info column, 36px buttons, larger fonts.
• size="large": a full-bleed banner with a 64px cover, 400px info column, 44-56px buttons, the largest fonts.

On viewports narrower than 768px, all three variants collapse to the same compact layout. Unknown values fall back to small. The attribute is purely visual; persistence, playback, and the audio source URL are unaffected.

Changes

• assets/js/podcast-player.js:
  • PodcastFooter.observedAttributes += "size"
  • attributeChangedCallback adds a size no-op branch (CSS handles the visual change)
  • Shadow DOM style: two new :host([size="..."]) blocks declare 13 --podcast-footer-* custom properties each
  • Existing rules updated to consume the new vars with original hardcoded values as fallbacks (default = byte-equivalent to today)
  • Mobile media query extended to override the size vars for both medium and large, so all three sizes collapse to the compact layout on narrow viewports
• exampleSite/layouts/_default/baseof.html: size="medium" added to the default footer to demonstrate the feature
• exampleSite/content/docs/homepage-setup.md (+ ES mirror): new ## Footer Size H2 section
• CHANGELOG.md: bullet under [Unreleased] / ### Added
• tests/js/web-component.test.js: 13 new Vitest unit tests
• tests/e2e/player.spec.js: 6 new Playwright E2E tests (incl. cover-width assertions at each size + responsive collapse + Turbolinks persistence)

Test plan

• npx vitest run - 134/134 tests pass (121 pre-existing + 13 new)
• npx playwright test ... tests/e2e/player.spec.js - 34/34 tests pass (28 pre-existing + 6 new), including computed-style assertions at each size
• No em-dashes in any new doc or CHANGELOG content
• Manual smoke test against the example site recommended before merge

Closes #62