Fix Personal quality preset fidelity and mobile grading bay

[Jellman86]

What changed

• Resolve every anonymous video candidate from its complete named preset bundle: codec, container, HDR, video-audio codec/bitrate, and downmix policy.
• Expose those complete server-owned preset definitions through /api/library-options, and make the library slider restore the full selected bundle when moving away from Scott's Settings.
• Apply a revealed Personal quality result through the same preset defaults, removing the stale Scott-specific branch that still wrote Preserve HDR.
• Refactor the comparison viewer into a width-driven grading stage so video and images cannot exceed narrow portrait or landscape cards; tighten the header hierarchy and make fullscreen target the semantic stage.
• Document the exact candidate/container behavior and update the Unreleased changelog.

Why

A library carrying Scott's Settings could leak its HDR tone-map, AAC 96 kbps, and stereo-downmix overrides into Conservative HEVC, AV1, and H.264 comparison candidates. That made nominally different candidates encode with the same effective bundle. Separately, aspect-video plus a fixed minimum height silently produced a 455 px player inside a 390 px viewport; the parent clipped it, so page-level overflow checks missed the defect.

Safe MP4-to-MKV fallback remains unchanged: retained image subtitles can still require MKV rather than being discarded.

User impact

Personal quality candidates are genuinely distinct and revealed recommendations persist the exact preset the user judged. The comparison player now remains fully visible and touch-friendly on phones, short landscape screens, and fullscreen.

Validation

• dotnet build Optimisarr.slnx --no-restore -warnaserror — 0 warnings, 0 errors
• dotnet test Optimisarr.slnx --no-build — 1,493 passed
• npm run check — 0 errors, 0 warnings; 8 locales audited
• npm run build — passed
• npx playwright test — 48 passed
• python3 scripts/check_docs.py — passed
• python3 scripts/check_openapi.py --configuration Release --no-build — up to date
• Visual QA at 390×844 portrait, 812×375 landscape, and 1440×900 desktop