Move Dockhand container management to its own top-level Docker page

Container management was buried inside the generic Integrations
browsing view, mixed in with five unrelated integration types behind
a single "pick any integration" dropdown. Give it a dedicated page,
matching how Servers & Tasks and DNS already get their own top-level
spot instead of living inside Integrations.

New web/src/pages/Docker.tsx reuses the existing, unchanged Dockhand
API routes and adapter (no server changes) — it just has its own
integration picker scoped to Dockhand only, rather than sharing
Integrations' any-type dropdown. Added a Docker nav item (right after
Integrations) and route.

Removed the Dockhand-specific state/handlers/table from Integrations
entirely; adding, editing, enabling/disabling, and deleting the
Dockhand credential itself still happens under Integrations -> Manage
integrations like every other integration. Selecting a Dockhand row in
Integrations' generic browsing dropdown now points to the Docker page
instead of rendering a container table there too.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

README.md

@@ -39,9 +39,12 @@ All modules from the original plan are built:
 - **Integrations → Gitea** — repo list with each repo's last CI run status,
   and re-running just the failed jobs in a run; a live repo-count widget
   (with a failing-build warning).
-- **Integrations → Dockhand** — container status across every Docker host
-  Dockhand manages (one credential covers all of them), with
-  start/stop/restart actions; a live running/total widget.
+- **Docker** (its own top-level page, backed by the Dockhand
+  integration) — container status across every Docker host Dockhand
+  manages (one credential covers all of them), with start/stop/restart
+  actions and a live running/total widget. Add/edit/remove the
+  Dockhand credential itself under Integrations → Manage integrations
+  as with every other integration.
 - **Integrations → Semaphore** — Ansible run status per template across
   every project, with a "Run" action to trigger a template; a live
   template-count widget (with a last-failed warning).