feat(AppShell): add Included Library upload flow, sync asset/element deletes

[alexwarren]

Summary

• Included Library elements had no proper "add" flow — the manual file picker was broken (<source>libraries</source> in CoreEditorIncludedLibrary.aslx didn't match any real extension), so the only affordance was creating a blank element and hoping the property panel worked. New AddLibraryModal (upload-only, since a library can only ever come from an existing file, unlike Javascript's "type a new name" option) plumbs a filename through at creation time, matching the JS element's lockedaftercreate shape.
• Deleting a Javascript/Included Library element now cascades to delete its underlying asset too (with a confirmation, since asset deletes live outside Quest's undo system and are permanent) — previously the file was silently orphaned. The reverse direction also works: deleting the file from the Asset Manager now removes the owning element instead of leaving it pointing at a missing file.
• Found and fixed a related bug along the way: LocalDraftAdapter.listAssets() blanket-excluded every .aslx file (to hide the game's own file), which also hid library files — silently breaking both the delete cascade and the "pick an existing library" picker. Narrowed the exclusion to just the draft's own filename, since that OPFS directory is exclusively app-owned. Left BrowserFileAdapter/ElectronAdapter as-is, since a real folder can legitimately contain unrelated .aslx files.
• undo()/redo() now detect when Quest's own undo restores an element whose file was cascade-deleted outside that system (undo only rewinds the WorldModel transaction, not the out-of-band asset delete) and toast a warning instead of leaving a silently broken element sitting in the tree.

Test plan

• dotnet build --configuration Release — clean
• dotnet test tests/EditorCoreTests — all passing, including updated IncludedLibraryTests
• svelte-check / eslint in src/AppShell — clean
• Manually verified in-browser (WasmEditor Debug build + AppShell dev server): add-library-from-upload, add-library-from-existing-picker, delete-with-confirm cascading the asset, cancel preserves both, Asset-Manager-side delete removing the owning element, and the undo-after-cascade-delete toast — all working; built-in libraries (Core.aslx/English.aslx) remain protected throughout