fix: downgrade Flutter to 3.44.0 — cirruslabs image for 3.44.1 not published (#409)

## Summary

- Reverts `.fvmrc` from `3.44.1` to `3.44.0`
- `ghcr.io/cirruslabs/flutter:3.44.1` returns "manifest unknown" — image does not exist on GHCR
- `ghcr.io/cirruslabs/flutter:3.44.0` is confirmed present — CI can pull the toolchain container again

Closes #408

## Test plan

- [x] `docker manifest inspect ghcr.io/cirruslabs/flutter:3.44.0` returns a valid manifest (verified locally)
- [x] `docker manifest inspect ghcr.io/cirruslabs/flutter:3.44.1` returns "manifest unknown" (confirmed root cause)
- [ ] CI pipeline should pass once the toolchain image resolves correctly

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Thomas SharedInbox <sharedinbox@thomas-guettler.de>
Reviewed-on: https://codeberg.org/guettli/sharedinbox/pulls/409

.fvmrc

@@ -1,3 +1,3 @@
 {
-  "flutter": "3.44.1"
+  "flutter": "3.44.0"
 }