Separate Preferences and isolate scoped settings drafts

[VolkerChristian]

Settings previously edited the live talk while Save/Cancel remained open; conference and media operations could save those uncommitted edits. Scoped settings now edit independent drafts. Save replays once and persists once; Cancel leaves the talk and its file unchanged and removes unused session imports.

Preferences separates immediate computer controls from inheritable Talk defaults. Fixed-scope dialogs use schema-built pages and consistent source/Reset rows; Slide flow owns playback controls and Slide sound owns voice, delivery and clips. Existing settings resolution, language versions and audio fingerprints remain unchanged.

• Stacked on PR Speed up settings edits and add Editor/Presenter slide navigation #3; review/merge PR A first.
• Linux: complete suite passes on stock Qt 6.11.2 and native Qt 6.10.2/Breeze, 561 tests each. Private null sink; physical mute/volume/ports unchanged.
• 60-slide dialog edit: 12.998 → 0.340 ms, with zero main-window refreshes; Save: 18.242 → 12.693 ms. Approved two readiness passes retained.
• 130 screenshots inspected, including minimum sizes, light/dark and 150%; keyboard/pointer interaction tests pass.
• Production +106 net from PR A (ceiling +150); tests +344 net. Maximum settings-method CC 18.

See the review report for commands, benchmark data, acceptance coverage, failed attempts, screenshots and accounting. GPU startup and live Wayland verification are outside this Linux UI/settings change.