Add user theme support and migrate frontend to HTMX templates

- Add theme column to users table with default 'tokyo-night'
- Update all user queries to include theme field
- Add ListUsers and UpdateUserTheme database queries
- Update auth handlers to support HTMX form submissions and JSON API
- Add ListUsers API endpoint
- Replace embedded static files with Go templates
- Update Dockerfile to copy templates directory
- Redesign index.html with inline styles and HTMX forms
- Update Bruno API testing requests for auth endpoints

backend/Dockerfile

@@ -15,6 +15,9 @@ RUN cd backend && go mod tidy
 # Generate sqlc code
 RUN cd backend && sqlc generate
 
+# Copy templates (updated for new homepage)
+COPY backend/templates ./backend/templates
+
 # Build the application
 RUN cd backend && CGO_ENABLED=0 GOOS=linux go build -a -installsuffix cgo -o ../main ./cmd/server
 
@@ -30,6 +33,9 @@ COPY --from=builder /app/main .
 # Copy migrations (if needed for initialization)
 COPY --from=builder /app/backend/migrations ./migrations
 
+# Copy templates
+COPY --from=builder /app/backend/templates ./templates
+
 # Expose port
 EXPOSE 8765