Add landing page, redesign emails, and return updated task on completion

- Integrate landing page into Go app (served at root /)
- Add STATIC_DIR config for static file serving
- Redesign all email templates with modern dark theme styling
- Add app icon to email headers
- Return updated task with kanban_column in completion response
- Update task DTO to include kanban column for client-side state updates

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Dockerfile

@@ -58,6 +58,9 @@ COPY --from=builder /app/worker /app/worker
 # Copy templates directory
 COPY --from=builder /app/templates /app/templates
 
+# Copy static landing page files
+COPY --from=builder /app/static /app/static
+
 # Copy migrations and seeds for production use
 COPY --from=builder /app/migrations /app/migrations
 COPY --from=builder /app/seeds /app/seeds
@@ -116,6 +119,9 @@ COPY --from=builder /app/worker /app/worker
 # Copy templates directory
 COPY --from=builder /app/templates /app/templates
 
+# Copy static landing page files
+COPY --from=builder /app/static /app/static
+
 # Copy migrations and seeds
 COPY --from=builder /app/migrations /app/migrations
 COPY --from=builder /app/seeds /app/seeds