fix(atlas): constrain bucket list width to prevent panel overflow

With 30+ bucket list entries the panel expanded to near-full viewport
width, elongating the Stats tab, hiding overflow entries, and covering
the Leaflet zoom controls. Measure the stats content width via
ResizeObserver and use it as maxWidth on the horizontal bucket row so
scroll activates exactly when entries exceed the stats panel width.

Also fixes the ResizeObserver test mock to use a class (matching the
IntersectionObserver pattern) so the instance methods are accessible.

Closes #787

client/tests/setup.ts

@@ -64,11 +64,13 @@ class _MockIntersectionObserver {
 globalThis.IntersectionObserver = _MockIntersectionObserver as unknown as typeof IntersectionObserver;
 
 // ResizeObserver — used by resizable panels
-globalThis.ResizeObserver = vi.fn().mockImplementation(() => ({
-  observe: vi.fn(),
-  unobserve: vi.fn(),
-  disconnect: vi.fn(),
-})) as unknown as typeof ResizeObserver;
+class _MockResizeObserver {
+  observe = vi.fn()
+  unobserve = vi.fn()
+  disconnect = vi.fn()
+  constructor(_callback: ResizeObserverCallback) {}
+}
+globalThis.ResizeObserver = _MockResizeObserver as unknown as typeof ResizeObserver;
 
 // URL.createObjectURL / revokeObjectURL — Node 22 URL.createObjectURL requires
 // a native node:buffer Blob; passing a jsdom Blob throws ERR_INVALID_ARG_TYPE.