test(ui-lab): add extreme mock scenario

Add an extreme UI Lab scenario with virtual-scale library, album, cluster, and tag counts while keeping the rendered media fixture set manageable.

Return the full extreme tag set from the mock backend so tag manager layouts can be stress-tested with 10k-100k tag counts.

src/dev/mockScenarios.ts

@@ -1,4 +1,4 @@
-export type MockScenario = "rich" | "empty" | "busy" | "duplicates" | "album" | "errors" | "huge";
+export type MockScenario = "rich" | "empty" | "busy" | "duplicates" | "album" | "errors" | "huge" | "extreme";
 
 const SCENARIOS = new Set<MockScenario>([
   "rich",
@@ -8,6 +8,7 @@ const SCENARIOS = new Set<MockScenario>([
   "album",
   "errors",
   "huge",
+  "extreme",
 ]);
 
 export function getMockScenario(): MockScenario {