Redesign README product presentation and previews

[debpalash]

The README buried installation beneath several large demos and repeated its opening copy. This redesign puts installation first and gives the page six clear sections, with optional platform instructions, demos, and developer notes in expandable panels.

• Introduce a matte mineral palette and consistent frames across the hero, screenshots, and GIFs.
• Add a dedicated phone hero and still previews selected by reduced-motion preferences.
• Preserve real app captures, original MP4 links, platform caveats, source-build requirements, and media credits.

Desktop hero · Phone hero

Validation:

• zig build test: passed, 0 failures.
• python3 tests/test_features.py: 481 passed, 0 failed, 2 warnings, 13 skipped.
• Existing README-related playback-version and phone-pairing checks: 2 passed, 0 failed.
• 32 local links/assets, navigation anchors, node --check, and git diff --check: passed.
• GitHub Markdown rendering, desktop/light/dark previews, 375 px phone layout with all panels expanded, and responsive/reduced-motion image selection: checked.

Documentation and its artwork renderer only; no app runtime changes or new runtime tests.