release: v0.2 – collaboration docs, richer demo data, new screenshots

- Turn the collaboration plan into module docs (Doku/modules/collaboration.md)
  describing the implemented state; remove Doku/plans
- Header version comes from the build: git tag via APP_VERSION (CI passes
  it for tag builds), otherwise "<package version>-dev"; package 0.2.0
- Sample data (setup option and demo mode): six inbox thoughts, two task
  lists with Kanban states, subtasks and due dates, shopping and packing
  lists, three notes in folders incl. a long nested outline; demo mode
  shares the note and two lists with user/user
- e2e/screenshots.sh renews the README screenshots from demo data
- README: live collaboration, editor sharing, image tags, version badge

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Dockerfile

@@ -6,7 +6,9 @@ WORKDIR /app/frontend
 COPY frontend/package*.json ./
 RUN npm install --no-audit --no-fund
 COPY frontend/ ./
-RUN npx tsc --noEmit && npm run test:run && npm run build
+# Version shown in the header: the git tag for releases (set by CI), otherwise "<package version>-dev"
+ARG APP_VERSION=""
+RUN npx tsc --noEmit && npm run test:run && APP_VERSION="${APP_VERSION}" npm run build
 
 # ==============================================================================
 # Stage 2: Go Backend Lint, Unit Tests & Binary Build (Cached Go module & sqlite layer)