build: switch from devenv to nix flakes with uv2nix

- Replace devenv setup with pure nix flakes using uv2nix, pyproject-nix,
  pyproject-build-systems, and git-hooks.nix (adapted from hokusai)
- Add forgejo CI pipeline with lint/check/test and build jobs
- Wrap cv binary with LD_LIBRARY_PATH for weasyprint native dependencies
- Move basedpyright to dev dependency group, add pytest and ruff
- Fix package imports to use hatch source remapping (src/ layout)
- Update .envrc to use flake, expand .gitignore

BREAKING CHANGE: devenv is no longer used, run direnv allow to reload

.forgejo/workflows/ci.yml

@@ -0,0 +1,27 @@
+name: Continuous Integration
+
+on:
+  push:
+  workflow_dispatch:
+
+jobs:
+  check:
+    name: Lint, Check & Test
+    runs-on: nix
+
+    steps:
+      - name: Check out Repository
+        uses: https://git.konstantinfickel.de/actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6
+
+      - run: nix --version
+      - run: nix flake check
+
+  build:
+    name: Build Package
+    runs-on: nix
+
+    steps:
+      - name: Check out Repository
+        uses: https://git.konstantinfickel.de/actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6
+
+      - run: nix build