Lock in the fix from 31b73460c3 (#11803) with a regression
test for the exact reported scenario: loading the viewer
with a URL that already contains `zoom=fill`.
At 2.18.0-RC5 `update-zoom-querystring` navigated without
any comparison, so the load sequence bundle-fetched →
zoom-to-fill → update-zoom-querystring → nav → navigated
re-ran forever and crashed the page with React error #185
("maximum update depth exceeded"). The guard added in
31b73460c3 breaks the cycle; the new test asserts that a
bundle fetch against a `zoom=fill` route emits no
navigation events.
Also updates the dashboard/viewer frontend memory to
document the guard and the loop it prevents.
AI-assisted-by: glm-5.3-flash
Clamp the frame index to the valid range in zoom-to-fit and
zoom-to-fill events before accessing the frames vector. When the
URL query parameter :index exceeds the number of frames on the
page (e.g. index=1 with a single frame), nth would throw
"No item 1 in vector of length 1". Also adds unit tests covering
the boundary condition.
