Add selected upstream media and quality-of-life improvements

[LucifersCircle]

Summary

Integrates the recommended and optional low-conflict improvements selected from open redlib-org/redlib pull requests:

• fix(video): add support for CMAF video format alongside DASH redlib-org/redlib#506 — support CMAF video URLs alongside DASH
• Lazy load post images redlib-org/redlib#539 — lazy-load post images
• Fix share links for a post on a user's profile redlib-org/redlib#560 — handle user-profile post share links
• Fix default post sort setting not applied until settings are saved redlib-org/redlib#521 — respect the configured default post sort
• Prevent an out of bounds from trying to access comments[0].author.name when there are no comments redlib-org/redlib#542 — guard empty comment threads
• Increase touch area of comment collapse button on mobile redlib-org/redlib#520 — enlarge the mobile comment-collapse touch target
• feat: collapse comment by clicking on indent line redlib-org/redlib#291 — collapse comments by selecting the indent line
• feat: proxy and embed giphy gifs in comments redlib-org/redlib#561 — proxy and embed Giphy links in comments
• Improve searchbox outline when focusing redlib-org/redlib#524 — improve the search focus outline
• fix: only autoplay videos if visible redlib-org/redlib#194 — autoplay HLS video only while visible
• gallery download redlib-org/redlib#546 — download all images in a gallery
• Fix: Footer at a fixed position (bottom of the screen) redlib-org/redlib#394 — keep the footer fixed to the viewport bottom

The imported search CSS's trailing whitespace was removed. The existing RSS code was formatted and six current Clippy findings were corrected without changing their intended behavior. No rate-limit-resilience behavior was replaced.

Verification

Passed:

• Three-way merge checks against the fork's current main
• Combined application of all twelve selected PRs
• GitHub Docker workflow: cargo check --locked --all-targets
• GitHub Docker workflow: all deterministic resilience tests
• cargo fmt --all -- --check
• cargo clippy -- -D warnings
• git diff --check
• node --check static/downloadGallery.js
• node --check static/playHLSVideo.js

Known upstream/network-dependent failure:

• The full cargo nextest run job fails only at oauth::tests::test_generic_web_backend, which makes a live Reddit request.
• It failed all configured retries on two workflow attempts.
• This bundle does not change src/oauth.rs or that backend. The deterministic OAuth, client, compile, formatting, and Clippy checks pass.
• Docker publishing is correctly skipped for pull requests; merging to main will trigger the normal multi-platform image publication.