Add Nix flake, .envrc, Taskfile, and Stalwart dev server

- flake.nix: Flutter 3.41.6, Android SDK, Stalwart, GTK3/build
  tools for Linux desktop, go-task
- .envrc: copied from sharedinbox — use flake + dotenv_if_exists
- Taskfile.yml: analyze, test, integration, codegen, run tasks
- stalwart-dev/: IMAP+SMTP dev server reused from sharedinbox
- test/integration/imap_sync_test.dart: login, list mailboxes,
  send via SMTP and receive via IMAP
- pubspec.yaml: add flutter_secure_storage

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

.envrc

@@ -0,0 +1,12 @@
+# shellcheck shell=bash
+
+# .envrc file of direnv.
+# If you use vsode, pleaes use the `direnv` extension.
+
+# Use nix-direnv
+# https://github.com/nix-community/nix-direnv
+# Ensures that flake.nix gets evaluated.
+use flake
+
+# Load variables from .env
+dotenv_if_exists