PMM-15326: Rename the UI's "PSMDB OpenManager" to "OpenManager"

Two user-visible strings: the sidebar's top-level entry and the Overview page's
heading. The comment in `PomHeader` describing that nav entry follows them, so it does
not name something the reader cannot find.

The remaining "PSMDB Open Manager" occurrences are deliberately left. They gloss where
the acronym POM comes from - in `router.tsx`, `lib/constants.ts` and the workspace
docs - and that origin does not change because the displayed name got shorter.
Renaming them would leave "POM" unexplained.

No test pinned the old string, and none needed updating.

Signed-off-by: Pawel Lebioda <pawel.lebioda@percona.com>

Author: plebioda

ui/apps/pmm/src/contexts/navigation/navigation.utils.tsx

@@ -331,7 +331,7 @@ export const addSepApps = (): NavItem[] => [
 export const addPom = (): NavItem[] => [
   {
     id: 'pom',
-    text: 'PSMDB OpenManager',
+    text: 'OpenManager',
     icon: MongoIcon,
     url: POM_PATH,
     matches: [POM_PATH],

ui/packages/plugins/pom/src/OverviewPage.tsx

@@ -403,7 +403,7 @@ export function OverviewPage() {
     <Stack gap={3}>
       <Stack gap={1}>
         <PomHeader
-          title="PSMDB OpenManager"
+          title="OpenManager"
           subtitle={
             <Typography variant="body2" color="text.secondary">
               Every monitored MongoDB cluster, one table per environment. Unfold

ui/packages/plugins/pom/src/components/PomHeader.tsx

@@ -21,7 +21,7 @@ import { Stack, Typography } from '@mui/material';
  * Page title, subtitle, and a slot for page-level actions.
  *
  * Navigation between Overview and Discovery lives in PMM's sidebar, under the
- * expandable PSMDB OpenManager entry — so this deliberately renders no tabs.
+ * expandable OpenManager entry — so this deliberately renders no tabs.
  */
 export function PomHeader({
   title,