fix: downgrade Flutter to 3.44.0 — cirruslabs image for 3.44.1 not published (#428)

## Summary

- Downgrades `.fvmrc` from Flutter `3.44.1` back to `3.44.0` — `ghcr.io/cirruslabs/flutter:3.44.1` does not exist on GHCR so every Dagger-based deploy job fails with "not found"
- Extends `scripts/check_ci_images.sh` to also validate the Flutter image derived from `.fvmrc` (previously only literal `From("...")` calls in `ci/main.go` were checked, so Renovate bumps to non-existent images went undetected)
- Updates `.pre-commit-config.yaml` to trigger the `ci-image-exists` hook on `.fvmrc` changes as well as `ci/main.go`

## Root cause

Recent run logs showed:
```
! ghcr.io/cirruslabs/flutter:3.44.1: not found
Error: failed to resolve image "ghcr.io/cirruslabs/flutter:3.44.1"
```
Renovate bumped Flutter to 3.44.1 (#411) but cirruslabs has not published that image — the latest available is `3.44.0`. Same root cause as #409, but the pre-commit guard only watched `ci/main.go`, not `.fvmrc`.

Closes #427

Co-authored-by: Thomas SharedInbox <sharedinbox@thomas-guettler.de>
Reviewed-on: https://codeberg.org/guettli/sharedinbox/pulls/428

.fvmrc

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

.pre-commit-config.yaml

@@ -47,4 +47,4 @@ repos:
         language: system
         entry: bash -c 'cd "$(git rev-parse --show-toplevel)" && nix develop --command task check-ci-images'
         pass_filenames: false
-        files: ^ci/main\.go$
+        files: ^(ci/main\.go|\.fvmrc)$

scripts/check_ci_images.sh

@@ -6,7 +6,18 @@ set -euo pipefail
 ROOT=$(git rev-parse --show-toplevel)
 FILE="$ROOT/ci/main.go"
 
-images=$(grep -oP 'From\("\K[^"]+' "$FILE" | sort -u)
+# Static images from From("...") literals in ci/main.go
+static_images=$(grep -oP 'From\("\K[^"]+' "$FILE" | sort -u)
+
+# Dynamic Flutter image derived from .fvmrc (not a literal in main.go)
+FVMRC="$ROOT/.fvmrc"
+flutter_version=$(python3 -c "import json; print(json.load(open('$FVMRC'))['flutter'])" 2>/dev/null || true)
+flutter_image=""
+if [ -n "$flutter_version" ]; then
+  flutter_image="ghcr.io/cirruslabs/flutter:$flutter_version"
+fi
+
+images=$(printf '%s\n%s\n' "$static_images" "$flutter_image" | grep -v '^$' | sort -u)
 
 if [ -z "$images" ]; then
   echo "check-ci-images: no From() image references found in $FILE"