fix: upload to Android-invalid folder paths

[sgallese]

Summary

• Fix uploads into server folders whose names contain characters Android cannot use in local path segments.
• Sanitize only the temporary local upload path derived from the remote path; the remote destination path is unchanged.
• Validate local download paths segment-by-segment before creating Android files.

Fixes #14711.

Testing

Manual validation against a local Nextcloud 31 server:

• Created a server folder named test <3.
• Logged into the Android app and opened that folder.
• Uploaded nextcloud-pr7-video-upload.txt from local device storage into test <3.
• Confirmed the uploaded file appeared in the app alongside the seed file.
• Verified the uploaded file over WebDAV and checked the downloaded content matched the local file.

🖼️ Screenshots

Uploaded file inside test <3 after the fix:

🎥 Videos

User-flow upload into test <3 (21 seconds):

nextcloud-user-upload-14711-short2.mp4

🏁 Checklist

• ⛑️ Tests (unit and/or integration) are included or not needed
• 🔙 Backport requests are created or not needed: /backport to stable-xx.x
• Milestone: maintainers can add one if needed.
• 🌸 PR title is meaningful (if it should be in the changelog: is it meaningful to users?)

🤖 AI (if applicable)

• The content of this PR was partly or fully generated using AI