Desktop: Add an attribute to identify notes imported from OneNote

[Rygaa]

Add an attribute to each note imported from OneNote to record its source. This would let Joplin identify these notes directly without relying on HTML class names and apply OneNote specific handling only to them.

It would help us in future bugs related to OneNote like #16424 and #15578

Note: We should create a new attribute and not try to reuse an existing one. Also we should not try to support old notes

[Rygaa]

@personalizedrefrigerator I checked the code but I’m not sure where to store the new OneNote key

• A new database column: We can read it directly from the note without checking the HTML. It stays separate from the note content but it needs a database migration.

• A marker in the note content: No migration is needed. But the HTML sanitizer removes <meta> tags and Markdown conversion removes normal HTML comments. A data-* attribute might work but I still need to test whether editing keeps it.

• A new key inside user_data: We can use the existing metadata storage without changing the database or note content. We already has helpers for this. Reading the key needs JSON parsing instead of reading a column directly.

I think user_data could be a good fit. What do you think?