fix: bump Dagger Flutter image from 3.44.0 to 3.44.1 to match .fvmrc

.fvmrc was updated to Flutter 3.44.1 in d7a9c2b but ci/main.go still
referenced 3.44.0. The version mismatch means the Dagger toolchain used
by both ci.yml and deploy.yml builds the app with a different Flutter SDK
than the one used locally. This can cause build or format failures.

Also update the stale 3.41.6 version in the Graph() diagram.

Closes #394

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ci/main.go

@@ -181,7 +181,7 @@ func New(
 // Used as the base for pubGetLayer so flutter pub get is execution-cached between runs.
 func (m *Ci) toolchain() *dagger.Container {
 	return dag.Container().
-		From("ghcr.io/cirruslabs/flutter:3.44.0").
+		From("ghcr.io/cirruslabs/flutter:3.44.1").
 		WithExec([]string{"apt-get", "-qq", "update"}).
 		WithExec([]string{"apt-get", "install", "-y", "-qq", "clang", "cmake", "ninja-build", "pkg-config", "libgtk-3-dev", "liblzma-dev", "libsecret-1-dev", "libgcrypt20-dev", "libjsoncpp-dev", "sqlite3", "iproute2", "netcat-openbsd", "xvfb", "libosmesa6", "libegl1", "lld"}).
 		WithExec([]string{"useradd", "-m", "-s", "/bin/bash", "ci"}).
@@ -902,7 +902,7 @@ func (m *Ci) Graph() string {
 ` + "```" + `mermaid
 flowchart TD
     subgraph dagger ["Dagger · Check pipeline"]
-        toolchain["toolchain\nflutter:3.41.6 + NDK + apt + precache"]
+        toolchain["toolchain\nflutter:3.44.1 + NDK + apt + precache"]
         pubGet["pubGetLayer\nflutter pub get"]
         codegen["codegenBase\nbuild_runner build\n(shared cache)"]
         stalwart(["Stalwart service\nIMAP · JMAP · SMTP · Sieve"])