file browser crashes on Firefox/Safari, the "fallback" is just a toast saying use Chrome

[avinxshKD]

the directory browser (showDirectoryPicker), file save (createWritable), and save-as (showSaveFilePicker) all use the File System Access API which only exists in Chrome and Edge. on Firefox/Safari these throw a TypeError and the user gets nothing.

the "fallback" is literally toast.info('Switch to Edge/Chrome!'), that's it.

What breaks on Firefox:

1. clicking "Open Directory" in file browser => crash
2. clicking "Save" in file editor => crash
3. clicking "Save As" in file editor => the file-saver fallback exists but hardcodes MIME to application/json for all file types
4. saving a .graphml from toolbar => crash

Bonus problems while we're at it:

file extension blocklist is case-sensitive: .PNG and .JPEG pass through
no file size limit. drop a huge file and the tab freezes
File objects get JSON.stringify'd to localStorage as {} (line 42 of fileBrowser) ,they don't serialize

[avinxshKD]

@pradeeban what do you think? plan is to abstract it into a fileSystemService.js that handles feature detection once and keeps the fallback logic out of the components

[pradeeban]

@avinxshKD we officially only support Chrome. The file save and backend executions rely heavily on Chrome stuff. So, it makes sense it fails on Firefox or Safari.

Of course, we have to "fail" graciously. Is that a nice dialogue that you are seeing? If so, all good.

I have not tested concore-editor on anything other than Chrome lately.

Please create separate PRs for other "bonus problems" such as the case sensitiveness as you noted.

[avinxshKD]

@pradeeban makes sense, save/save-as already fail graciously but open directory and open file picker have no try/catch at all on Firefox, they just throw silently. I'll add the browser check before those calls.
For bonus problems that I listed I'll do in a separate PR, lmk if the scope looks right :p