fix(#29 review r1): stack-delete leak, rollback/retention/version tests, UX trap

F3: deleting a file-based stack now removes the stack ROOT (compose/{id}) via a
new removeStackProjectDir helper, not stack.ProjectPath (which the PR repointed to
compose/{id}/v{N}) — old version dirs + parent no longer leak. Git stacks unchanged.
F1: tests for validateRollbackTarget (rejects 0/neg/>current/hole) and the rollback
snapshot (client content ignored, target read from disk, monotonic new version, note).
F2: tests for pruneStackFileVersionDirs (deletes given dirs, swallows errors) + the
post-commit gate contract + a monotonic-version regression guard.
F4: handler tests for ?version= (negative/out-of-range -> 400, valid version served,
legacy fallback).
F5: swagger @param version on GET file; @version 2.44.0 (handler.go) + package.json
2.44.0, matching APIVersion.
F6: the version selector no longer sets rollbackTo for the current/top version and
clears it on a manual buffer edit (so edits are honored, not silently discarded);
returning to the current version restores the current content. Distinguishes real
user edits from the programmatic version-load (CodeMirror ExternalChange).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

api/http/handler/handler.go

@@ -79,7 +79,7 @@ type Handler struct {
 }
 
 // @title PortainerCE API
-// @version 2.43.0
+// @version 2.44.0
 // @description.markdown
 // @x-tagGroups [{"name":"Access Control","tags":["auth","roles","team_memberships","teams","users"]},{"name":"Administration","tags":["backup","ldap","motd","settings","status","system","ssl","upload"]},{"name":"Docker","tags":["templates","custom_templates","docker","registries","resource_controls","stacks","webhooks","websocket"]},{"name":"Edge Compute","tags":["edge_agent","edge_groups","edge_jobs","edge","edge_stacks"]},{"name":"Environment Management","tags":["endpoint_groups","endpoints","tags"]},{"name":"GitOps","tags":["gitops"]},{"name":"Kubernetes","tags":["helm","kubernetes"]}]
 // @termsOfService