ci: automate dev container build via devcontainer.json + workflow (#553)

Closes #552

## Summary
- Add `.devcontainer/devcontainer.json` pointing at `../Dockerfile.dev` so VS Code / Codespaces / any devcontainer-aware tool can build the dev environment directly from source.
- Add `.forgejo/workflows/publish-dev-container.yml` that rebuilds `Dockerfile.dev` and pushes it to `codeberg.org/guettli/sharedinbox-dev` whenever `Dockerfile.dev`, the devcontainer config, or the workflow itself changes on `main`. The image is tagged both `:latest` and with the short commit SHA for pinnable references.
- The workflow uses the built-in `FORGEJO_TOKEN` to log in to Codeberg's container registry — no extra secrets required.

## Notes
- No existing references to `ghcr.io/guettli/sharedinbox-dev` were found in the repo, so issue step 3 (updating image references) is a no-op here.
- `workflow_dispatch` is also enabled so the image can be rebuilt manually if needed.

## Verification
- `python3 -c "import json; json.load(...)"` parses the devcontainer config.
- `python3 -c "import yaml; yaml.safe_load(...)"` parses the workflow.
- Triggers (paths filter) match the source files the issue identifies as drift risks.

Co-authored-by: Thomas Güttler <tilldu@googlemail.com>
Reviewed-on: https://codeberg.org/guettli/sharedinbox/pulls/553

.devcontainer/devcontainer.json

@@ -0,0 +1,10 @@
+{
+    "name": "SharedInbox Dev",
+    "build": {
+        "dockerfile": "../Dockerfile.dev",
+        "context": ".."
+    },
+    "workspaceFolder": "/src",
+    "workspaceMount": "source=${localWorkspaceFolder},target=/src,type=bind,consistency=cached",
+    "remoteUser": "ci"
+}