feature/allow user upload pdf and pictures as notes

[xingyi1145]

This pull request adds support for file attachments to study notes, allowing users to upload images or PDFs when creating a note. The backend now handles file uploads, saves files to an uploads directory, and exposes them via a static file route. The frontend has been updated to allow users to attach files when creating notes and to display/download attachments in the note detail view.

Backend: File upload support and API changes

• Added file upload handling to the create_note endpoint in notes.py, saving uploaded files to an uploads directory and storing their URLs in the database. ([src/backend/routes/notes.pyL18-R61](https://github.com/xingyi1145/SE-StudyCenter/pull/7/files#diff-3d3bed194cbe8022155d326655a4c127b5aa445cd54b3a380094249e5c06660aL18-R61))
• Updated the StudyNote model to include a new attachment_url field for storing the file URL. ([src/backend/models.pyR90](https://github.com/xingyi1145/SE-StudyCenter/pull/7/files#diff-765ee555033cbc1c0a66d54c0e9b9df7c4ce513b94fb9925e91aa18ee452b5feR90))
• Modified the API to serve files from the uploads directory using FastAPI's StaticFiles, ensuring the directory exists at startup. ([[1]](https://github.com/xingyi1145/SE-StudyCenter/pull/7/files#diff-7dc0ea0263421b070b14c289b52f9467996fb61796dbd1277b1c99ea1da14e4fR6-R8), [[2]](https://github.com/xingyi1145/SE-StudyCenter/pull/7/files#diff-7dc0ea0263421b070b14c289b52f9467996fb61796dbd1277b1c99ea1da14e4fR43-R48))

Frontend: File attachment UI and API integration

• Updated the note creation form to allow users to select and upload a file (image or PDF) and send it as part of the note creation request. ([[1]](https://github.com/xingyi1145/SE-StudyCenter/pull/7/files#diff-a44c4fdee246de97a005c7ad7bc90209e0513d60144b62939a1a7ed63d1c225fR188), [[2]](https://github.com/xingyi1145/SE-StudyCenter/pull/7/files#diff-a44c4fdee246de97a005c7ad7bc90209e0513d60144b62939a1a7ed63d1c225fR203-R208), [[3]](https://github.com/xingyi1145/SE-StudyCenter/pull/7/files#diff-a44c4fdee246de97a005c7ad7bc90209e0513d60144b62939a1a7ed63d1c225fR247-R256))
• Modified the API client to send note creation requests as multipart/form-data when a file is attached, and to handle the new attachment_url field. ([[1]](https://github.com/xingyi1145/SE-StudyCenter/pull/7/files#diff-eb8f64e76bf85ff0e9537eb54f79ae3c3a90d39267af822cbb236f69b07276bcR31-R42), [[2]](https://github.com/xingyi1145/SE-StudyCenter/pull/7/files#diff-eb8f64e76bf85ff0e9537eb54f79ae3c3a90d39267af822cbb236f69b07276bcR147-R157))

Frontend: Attachment display in note details

• Enhanced the note detail page to display the attachment if present: images are shown inline, and other files (like PDFs) have a download/view link. ([src/frontend/src/pages/NoteDetailPage.tsxR130-R164](https://github.com/xingyi1145/SE-StudyCenter/pull/7/files#diff-f8f0fb8ef6cc225bb55c005904c6f5c3c1e127e80c80bc8099b43637243fd709R130-R164))

Type and schema updates

• Updated backend and frontend types/schemas to include the optional attachment_url and file fields for study notes. ([[1]](https://github.com/xingyi1145/SE-StudyCenter/pull/7/files#diff-2a677c3ac994f65d3efddfbb7d8dacd43c233e9124b206c42ebd76f132451d0aR91), [[2]](https://github.com/xingyi1145/SE-StudyCenter/pull/7/files#diff-7fede23556481c8377d7d1bb0e61cd114eff01799a6c568da0b935c42bcc5532R65), [[3]](https://github.com/xingyi1145/SE-StudyCenter/pull/7/files#diff-7fede23556481c8377d7d1bb0e61cd114eff01799a6c568da0b935c42bcc5532R80))