Add CSRF protection to state-changing POST endpoints

Add gorilla/csrf middleware to protect all HTML-serving routes against
cross-site request forgery attacks. The webhook endpoint is excluded
since it uses secret-based authentication.

Changes:
- Add gorilla/csrf v1.7.3 dependency
- Add CSRF() middleware method using session secret as key
- Apply CSRF middleware to all HTML route groups in routes.go
- Pass CSRF token to all templates via addGlobals helper
- Add {{ .CSRFField }} / {{ $.CSRFField }} hidden inputs to all forms

Closes #11

go.mod

@@ -5,9 +5,11 @@ go 1.25
 require (
 	github.com/99designs/basicauth-go v0.0.0-20230316000542-bf6f9cbbf0f8
 	github.com/docker/docker v27.3.1+incompatible
+	github.com/docker/go-connections v0.6.0
 	github.com/go-chi/chi/v5 v5.2.3
 	github.com/go-chi/cors v1.2.2
 	github.com/google/uuid v1.6.0
+	github.com/gorilla/csrf v1.7.3
 	github.com/gorilla/sessions v1.4.0
 	github.com/joho/godotenv v1.5.1
 	github.com/mattn/go-sqlite3 v1.14.32
@@ -27,7 +29,6 @@ require (
 	github.com/containerd/log v0.1.0 // indirect
 	github.com/davecgh/go-spew v1.1.1 // indirect
 	github.com/distribution/reference v0.6.0 // indirect
-	github.com/docker/go-connections v0.6.0 // indirect
 	github.com/docker/go-units v0.5.0 // indirect
 	github.com/felixge/httpsnoop v1.0.4 // indirect
 	github.com/fsnotify/fsnotify v1.9.0 // indirect