Simplify Integrations to a plain list + Add button

Flagged back when Proxmox/Synology got their own pages: once all six
integration types had moved to dedicated pages, the "browsing"
dropdown here only ever showed one of six near-identical "go manage
this on its own page" redirects, and its final fallback branch was
dead code (every IntegrationType was already covered). Now that all
the actual data views have moved out, drop that mode entirely.

The page is now just what "Manage integrations" already was: a
sortable, CSV-exportable table of configured integrations (name/type/
status), visible to every role since none of that is sensitive, with
an admin-only "Add integration" button and per-row edit/enable-disable/
delete actions. No more mode toggle, no provider-picker dropdown, no
per-type redirect cards -- the six dedicated pages (and the sidebar
nav that already points at them) are how you actually use each
integration now.

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

README.md

@@ -80,8 +80,11 @@ All modules from the original plan are built:
     start/stop/restart actions, a host filter, and a live
     running/total widget.
 
-  Add/edit/enable/disable/remove each integration's credential itself
-  under Integrations → Manage integrations.
+  The Integrations page itself is now just a list of configured
+  integrations (name/type/status, visible to every role) with an
+  admin-only "Add integration" button and edit/enable/disable/delete
+  actions per row — the six dedicated pages above are where you
+  actually use each one.
 - Every table in the app is click-to-sort on any column (numbers, booleans,
   and dates/text sort correctly regardless of how the column formats them)
   and has an "Export CSV" button next to it that exports whatever's