Remove runtime nil checks for always-initialized components

Since signing_key is now required at config load time, sessMgr, encGen,
and signer are always initialized. Remove unnecessary nil checks that
were runtime failure paths that can no longer be reached.

- handlers.go: Remove conditional init, always create sessMgr/encGen
- auth.go: Remove nil checks for sessMgr
- imageenc.go: Remove nil check for encGen
- service.go: Require signing_key in NewService, remove signer nil checks
- Update tests to provide signing_key

internal/handlers/auth.go

@@ -15,13 +15,6 @@ import (
 // HandleRoot serves the login page or generator page based on authentication state.
 func (s *Handlers) HandleRoot() http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
-		// Check if signing key is configured
-		if s.sessMgr == nil {
-			s.respondError(w, "signing key not configured", http.StatusServiceUnavailable)
-
-			return
-		}
-
 		if r.Method == http.MethodPost {
 			s.handleLoginPost(w, r)
 
@@ -75,10 +68,7 @@ func (s *Handlers) handleLoginPost(w http.ResponseWriter, r *http.Request) {
 // HandleLogout clears the session and redirects to login.
 func (s *Handlers) HandleLogout() http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
-		if s.sessMgr != nil {
-			s.sessMgr.ClearSession(w)
-		}
-
+		s.sessMgr.ClearSession(w)
 		http.Redirect(w, r, "/", http.StatusSeeOther)
 	}
 }
@@ -87,7 +77,7 @@ func (s *Handlers) HandleLogout() http.HandlerFunc {
 func (s *Handlers) HandleGenerateURL() http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
 		// Check authentication
-		if s.sessMgr == nil || !s.sessMgr.IsAuthenticated(r) {
+		if !s.sessMgr.IsAuthenticated(r) {
 			http.Redirect(w, r, "/", http.StatusSeeOther)
 
 			return