refactor(ui/alerts): tighter inbox action bar, history uses global time range

Inbox: replace 4 parallel outlined buttons with 2 context-aware ones.
When nothing is selected → "Acknowledge all firing" (primary) + "Mark all
read" (secondary). When rows are selected → the same slots become
"Acknowledge N" + "Mark N read" with counts inlined. Primary variant
gives the foreground action proper visual weight; secondary is the
supporting action. No more visually-identical disabled buttons cluttering
the bar.

History: drop the local DateRangePicker. The page now reads
`timeRange` from `useGlobalFilters()` so the top-bar TimeRangeDropdown
(1h / 3h / 6h / Today / 24h / 7d / custom) is the single source of
truth, consistent with every other time-scoped page in the app.

ui/src/pages/Alerts/InboxPage.tsx

@@ -200,38 +200,47 @@ export default function InboxPage() {
           {allSelected ? 'Deselect all' : `Select all${rows.length ? ` (${rows.length})` : ''}`}
         </label>
         <span style={{ flex: 1 }} />
-        <Button
-          variant="secondary"
-          size="sm"
-          onClick={() => onBulkAck(selectedFiringIds)}
-          disabled={selectedFiringIds.length === 0 || ack.isPending}
-        >
-          Acknowledge selected
-        </Button>
-        <Button
-          variant="secondary"
-          size="sm"
-          onClick={() => onBulkAck(firingIds)}
-          disabled={firingIds.length === 0 || ack.isPending}
-        >
-          Acknowledge all firing
-        </Button>
-        <Button
-          variant="secondary"
-          size="sm"
-          onClick={() => onBulkRead(selectedIds)}
-          disabled={selectedIds.length === 0 || bulkRead.isPending}
-        >
-          Mark selected read
-        </Button>
-        <Button
-          variant="secondary"
-          size="sm"
-          onClick={() => onBulkRead(unreadIds)}
-          disabled={unreadIds.length === 0 || bulkRead.isPending}
-        >
-          Mark all read
-        </Button>
+        {selectedIds.length > 0 ? (
+          <>
+            <Button
+              variant="primary"
+              size="sm"
+              onClick={() => onBulkAck(selectedFiringIds)}
+              disabled={selectedFiringIds.length === 0 || ack.isPending}
+            >
+              {selectedFiringIds.length > 0
+                ? `Acknowledge ${selectedFiringIds.length}`
+                : 'Acknowledge selected'}
+            </Button>
+            <Button
+              variant="secondary"
+              size="sm"
+              onClick={() => onBulkRead(selectedIds)}
+              disabled={bulkRead.isPending}
+            >
+              Mark {selectedIds.length} read
+            </Button>
+          </>
+        ) : (
+          <>
+            <Button
+              variant="primary"
+              size="sm"
+              onClick={() => onBulkAck(firingIds)}
+              disabled={firingIds.length === 0 || ack.isPending}
+            >
+              Acknowledge all firing
+            </Button>
+            <Button
+              variant="secondary"
+              size="sm"
+              onClick={() => onBulkRead(unreadIds)}
+              disabled={unreadIds.length === 0 || bulkRead.isPending}
+            >
+              Mark all read
+            </Button>
+          </>
+        )}
       </div>
 
       {rows.length === 0 ? (