fix(security): convert scan history from full page to sheet overlay (#720)

Scan history is now a right-side sheet that layers over the current view
(typically Resources Hub) instead of a full-page activeView branch. The
sheet opens via the existing navigation event, fetches only when open,
dismisses on Escape or overlay click, and preserves the nested scan-details
and scan-compare sheets intact via Radix portal stacking.

The fetch effect now resets selection and page state on active-node change
exactly once, avoiding a double-fetch on node switches.

docs/features/vulnerability-scanning.mdx

@@ -285,7 +285,11 @@ Every scan Sencho runs is stored with its full vulnerability detail. Scan record
 - **Digest caching**: skip re-scanning an image that has already been scanned within 24 hours.
 - **Trend badges**: surface whether the latest scan added or resolved vulnerabilities compared to the previous scan for the same image.
 
-Open the **Scan history** page from the top of the Resources Hub to browse completed scans grouped by image, search by image reference, and pick two scans to compare.
+Click **Scan history** from the top of the Resources Hub to open a right-side sheet layered over the current page. The sheet lists completed scans grouped by image, lets you search by image reference, and lets you pick two scans to compare. Close the sheet by pressing Escape, clicking the overlay, or clicking the close button in the header.
+
+<Frame>
+  <img src="/images/vulnerability-scanning/scan-history-sheet.png" alt="Scan history sheet overlaid on the Resources Hub, with search box, pagination, and scan rows grouped by image" />
+</Frame>
 
 ## Comparing scans <Badge>Skipper</Badge>