Addressing Issue #10259

This PR fixes a bug in the file handling behavior of Excalidraw.
Previously, when a user opened a new .excalidraw file and then clicked Save, the app would still save to the previously opened file’s path.
This could accidentally overwrite the wrong file if the user wasn’t careful.

Changes made

• Updated logic in src/excalidraw-app/data/json.ts
• After opening a file via fileOpen(), the opened file’s handle is now correctly stored in appState.fileHandle
• This ensures subsequent saves target the newly opened file instead of the last one

Technical Explanation
The bug occurred because appState.fileHandle wasn’t being updated after loading a file from disk.
Now, inside loadFromJSON() (in json.ts), we assign:
return loadFromBlob(file, localAppState, localElements, file.handle);
so the handle gets preserved and used on the next save.

Testing Done

• Opened and saved multiple files in sequence.
• Verified correct overwrite behavior.
• No regression in other save/load behavior.