[WilliamRagstad]

Authors can create and rename rooms, import embedded PNG/JPEG/WebP backgrounds, draw rectangular hotspots in either direction, move them, adjust exact percentage bounds, and connect rooms through destination selectors. Hotspots have a keyboard-accessible list, default placement and arrow-key nudging. Coordinates remain relative to the original image when the canvas or background changes size.

Room links use stable IDs, so renames preserve them. Deletion retains references for repair: each validation entry identifies the affected object and field and selects its owner when clicked. Undo/redo restores authored data. Image imports cannot write into a different project opened while the file is loading; unsaved edits are guarded on navigation. Image bytes are shared between history snapshots to avoid duplicating large backgrounds for every edit.

Depends on #1 and targets its branch. The core-format/runtime dependency described there remains blocked; this PR works with the explicitly named draft contract.

Validation on this branch independently: eight Rust integration tests, a release WASM build, and the Chromium browser test passed. The browser test imports backgrounds, draws and nudges a hotspot, links rooms, follows a deleted-reference error, undoes deletion, saves/reopens with deep data equality, checks browser recovery, and rejects an unsupported version without replacing the project.