feat: An export user data button

[SeradedStripes]

what's this do?

Adds an export data feature to Stardance. (Closes #821)
Exported Data is put into a zip file and follows this format:

- `profile.json` - Your profile data
- `projects/` - Your projects, each containing:
  - `project.json` - Project metadata
  - `banner` - Project banner image (if uploaded)
  - `demo-video` - Demo video (if uploaded)
  - `devlogs/` - Development logs as Markdown files
    - `attachments/` - Images and files from each devlog

show it works

2026-07-16.17-54-20.mp4

ai?

Used opencode for planning the change