UI tweaks (#23492)

* slightly darken bg-card

* change menu label

* move snapshot retain out of advanced fields

* add new ui options for collapsibles

* backend title and description

* remove unused snapshot retention field

* update reference config

* remove further references to snapshots retain.mode

docs/docs/configuration/advanced/reference.md

@@ -655,11 +655,6 @@ snapshots:
   retain:
     # Required: Default retention days (default: shown below)
     default: 10
-    # Optional: Mode for retention. (default: shown below)
-    #   all - save all snapshots regardless of activity
-    #   motion - save snapshots for any detected motion
-    #   active_objects - save snapshots for active/moving objects
-    mode: motion
     # Optional: Per object retention days
     objects:
       person: 15

docs/docs/configuration/snapshots.md

@@ -111,7 +111,6 @@ Navigate to <NavPath path="Settings > Global configuration > Snapshots" />.
 | Field                                              | Description                                                                         |
 | -------------------------------------------------- | ----------------------------------------------------------------------------------- |
 | **Snapshot retention > Default retention**         | Number of days to retain snapshots (default: 10)                                    |
-| **Snapshot retention > Retention mode**            | Retention mode: `all`, `motion`, or `active_objects`                                |
 | **Snapshot retention > Object retention > Person** | Per-object overrides for retention days (e.g., keep `person` snapshots for 15 days) |
 
 </TabItem>
@@ -122,7 +121,6 @@ snapshots:
   enabled: True
   retain:
     default: 10
-    mode: motion
     objects:
       person: 15
 ```