feat: monolithic env var editing (bulk save, no per-var CRUD)

Replace individual env var add/edit/delete with a single bulk save
endpoint. The UI now shows a textarea with KEY=VALUE lines. On save,
all existing env vars are deleted and the full submitted set is
inserted.

- Replace HandleEnvVarAdd, HandleEnvVarEdit, HandleEnvVarDelete with
  HandleEnvVarSave
- Collapse 3 routes into single POST /apps/{id}/env
- Template uses textarea instead of per-row edit/delete forms
- No individual env var IDs exposed in the UI
- Extract parseEnvPairs helper to keep cyclomatic complexity low
- Use strings.SplitSeq per modernize linter
- Update tests for new bulk save behavior

closes #156
closes #163

internal/server/routes.go

@@ -82,10 +82,8 @@ func (s *Server) SetupRoutes() {
 			r.Post("/apps/{id}/stop", s.handlers.HandleAppStop())
 			r.Post("/apps/{id}/start", s.handlers.HandleAppStart())
 
-			// Environment variables
-			r.Post("/apps/{id}/env", s.handlers.HandleEnvVarAdd())
-			r.Post("/apps/{id}/env/{varID}/edit", s.handlers.HandleEnvVarEdit())
-			r.Post("/apps/{id}/env/{varID}/delete", s.handlers.HandleEnvVarDelete())
+			// Environment variables (monolithic bulk save)
+			r.Post("/apps/{id}/env", s.handlers.HandleEnvVarSave())
 
 			// Labels
 			r.Post("/apps/{id}/labels", s.handlers.HandleLabelAdd())