fix(playback): restore window placement after fullscreen

[RobDeFlop]

Summary

• replace fragile frontend fullscreen geometry handling with Windows WINDOWPLACEMENT restoration
• serialize fullscreen transitions and protect close/recording edge cases
• hide app chrome and background controls during fullscreen
• add keyboard and control behavior for fullscreen playback

Validation

• bunx tsc --noEmit
• bun run build
• cargo fmt --check
• cargo check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test --lib (37 passed)
• bun run tauri build --no-bundle

Manual Windows validation is still required for minimize/restore, maximized-window restoration, multiple monitors, and mixed DPI setups.

Fixes #10