Make preloading and shared cache opt-out Extended options

[AllTerrainDeveloper]

Window preloading and shared asset caching now start enabled site-wide, with administrator opt-out controls under Preferences → Features → Extended options. Both controls leave Beta features and apply when each shell reloads.

The existing runtime fields remain readable, but per-user values no longer control these features. updateOsSettings() ignores writes to the two mirrors, and resetting personal preferences preserves the site-wide values. Missing Extended option keys default to enabled; explicit opt-outs survive partial saves. The shared-cache filter still overrides the configured default. A migration note documents the scope and write-path change.

Validation:

• Full build and sync to the active Docker deployment on port 8889.
• JavaScript: 6,103 tests across 501 files passed.
• Relevant PHP suites: 105 tests / 287 assertions passed on both single-site and multisite.
• JavaScript lint, PHP lint, and TypeScript checks passed (existing file-length warnings remain).
• Browser verification on localhost:8889: both controls appear under Extended options, opt-outs persist after reload, and both were restored to enabled.