fix: address file browser bonus issues

[avinxshKD]

This PR addresses the bonus issues from #365 in a focused way.

Changes:
Made extension blocklist check case-insensitive (.PNG, .JPEG now blocked).
Added a file-size guard (10MB) before opening files to prevent UI freezes on huge files.
Stopped writing raw File objects to localStorage; now only serializable metadata is stored.

[avinxshKD]

Hey @pradeeban I kept this PR intentionally narrow.

Also If you prefer a different size threshold than 10MB, I can adjust it in this PR :p

[pradeeban]

@avinxshKD Pls address the above comment.

[avinxshKD]

@pradeeban Addressed, pls take a look

• Introduced MAX_FILE_SIZE_MB and derived byte limit from it.
• Switched both size-error messages to use that same constant.
• Added a short note in DEV-GUIDE.md on where to change the limit.

If you prefer this documented in another file, I can move it.

so sorry for the delay actually, was updating my proposal and handeling exams in parellel 😭

[pradeeban]

Looks good. Merging. Good luck with your exams.