Retain working file name and handle across sessions

[milanofthe]

Fixes #327.

Save always suggested the default file name after a reload: the autosave snapshot overwrote the model name with 'Autosave', and restoring a session never re-attached the file name or the File System Access handle.

• autosave keeps the working file's name in the snapshot metadata
• the current file reference (name + handle) is persisted in IndexedDB next to the autosave blob (same structured-clone mechanism as the recents list) and cleared on New
• restoring a session (editor prompt, landing preview, loadAutoSave) re-attaches the reference, so Save writes back to the original file on Chromium (after the permission re-prompt) and suggests the correct name in the download fallback on Firefox/Safari

Verified end-to-end: load model, reload, restore; the Save button targets the original file again.