Port worthwhile upstream fixes from Sep 1

[marcmy]

Targeted port of the worthwhile fixes currently on IridiumIO/master, adapted to the custom fork rather than merging upstream wholesale.

Ported

• Serialize database JSON updates/reads and validate downloaded JSON to prevent update/read races and corrupted-cache reads (ac08b088).
• Fix Watcher receiving BackgroundModeSelection changes on the wrong messenger type (34ebba95).
• Carry an already-paused background state into a compactor created during the pause race (c13a4bc3), while preserving the fork's watchdog/detached-native-task handling.
• Release analysis/estimation cancellation tokens in Finally paths (92682365).
• Unsubscribe FolderViewModel from global app-settings events when disposed (92682365).
• Fix HomeViewModel.RemoveFolder lifetime/selection handling, including the inverted stale-selection condition (92682365).
• Release disposed folder compressor/analyser/result/image references (92682365).
• Release the Analyser cache backing list on dispose without removing the fork's AnalysedFileDetails.FileInfo API (92682365 / conservative subset of c39b1fe3).
• Dispose BackgroundCompactor-owned analysers, preserve analyser lifetime for detached native tasks, restore the original process priority, and dispose the Process wrapper (020abe30), adapted to the fork's watchdog/detached-task architecture.

Deliberately not ported

• TOML/localization architecture migration and online language-update stack: broad churn with little benefit to this fork right now.
• Steam Monitor/Steam Library feature stack: substantial new UI/view-model/localization surface; better treated as a separate feature port.
• Upstream external-drive Watcher rewrite: this fork already has a more robust IsAvailable / recovery implementation.
• The rest of upstream's BackgroundCompactor rewrite: the fork's cancellation watchdog and detached-native-task ownership are more robust, so only the useful resource-ownership pieces were adapted.
• Forced GC.Collect() calls from the upstream memory-lifecycle commit.
• Removal of AnalysedFileDetails.FileInfo: potentially breaking API change; retained until its consumers are explicitly audited.

No project, localization, theme, or Steam-page files are touched by this PR.