refactor: use pinned golangci-lint Docker image for linting

Refactor Dockerfile to use a separate lint stage with a pinned
golangci-lint v2.11.3 Docker image instead of installing
golangci-lint via curl in the builder stage. This follows the
pattern used by sneak/pixa.

Changes:
- Dockerfile: separate lint stage using golangci/golangci-lint:v2.11.3
  (Debian-based, pinned by sha256) with COPY --from=lint dependency
- Bump Go from 1.24 to 1.26.1 (golang:1.26.1-bookworm, pinned)
- Bump golangci-lint from v1.64.8 to v2.11.3
- Migrate .golangci.yml from v1 to v2 format (same linters, format only)
- All Docker images pinned by sha256 digest
- Fix all lint issues from the v2 linter upgrade:
  - Add package comments to all packages
  - Add doc comments to all exported types, functions, and methods
  - Fix unchecked errors (errcheck)
  - Fix unused parameters (revive)
  - Fix gosec warnings (MaxBytesReader for form parsing)
  - Fix staticcheck suggestions (fmt.Fprintf instead of WriteString)
  - Rename DeliveryTask to Task to avoid stutter (delivery.Task)
  - Rename shadowed builtin 'max' parameter
- Update README.md version requirements

internal/handlers/profile.go

@@ -13,6 +13,7 @@ func (h *Handlers) HandleProfile() http.HandlerFunc {
 		requestedUsername := chi.URLParam(r, "username")
 		if requestedUsername == "" {
 			http.NotFound(w, r)
+
 			return
 		}
 
@@ -21,6 +22,7 @@ func (h *Handlers) HandleProfile() http.HandlerFunc {
 		if err != nil || !h.session.IsAuthenticated(sess) {
 			// Redirect to login if not authenticated
 			http.Redirect(w, r, "/pages/login", http.StatusSeeOther)
+
 			return
 		}
 
@@ -29,6 +31,7 @@ func (h *Handlers) HandleProfile() http.HandlerFunc {
 		if !ok {
 			h.log.Error("authenticated session missing username")
 			http.Error(w, "Internal server error", http.StatusInternalServerError)
+
 			return
 		}
 
@@ -36,17 +39,19 @@ func (h *Handlers) HandleProfile() http.HandlerFunc {
 		if !ok {
 			h.log.Error("authenticated session missing user ID")
 			http.Error(w, "Internal server error", http.StatusInternalServerError)
+
 			return
 		}
 
 		// For now, only allow users to view their own profile
 		if requestedUsername != sessionUsername {
 			http.Error(w, "Forbidden", http.StatusForbidden)
+
 			return
 		}
 
 		// Prepare data for template
-		data := map[string]interface{}{
+		data := map[string]any{
 			"User": &UserInfo{
 				ID:       sessionUserID,
 				Username: sessionUsername,