test(ui): split settings node suite by concern (#128788)

test(ui): split settings node suite by concern (#128788)

* test(ui): split settings node suite by concern

ui/src/app/settings.node.test.ts sat at 999 oxlint-counted lines against
the 1000-line max-lines cap, so any added assertion failed the core lint
shard — and, as in #128474, could pass locally while only the merge ref
tripped the cap.

Split the 42-test monolith along the surfaces it actually covers: the
base file keeps gateway URL derivation, tab-local token handling, and
per-gateway session scoping; preference persistence and layout
persistence move to dotted siblings, matching the existing
settings.sidebar-prefs.node.test.ts precedent. Test names and count are
unchanged (47 across the settings node suites, before and after).

The location/base-path/settings-fixture helpers and the storage
lifecycle hooks are now shared from ui/src/test-helpers/settings-node.ts
instead of being copy-pasted per file; sidebar-prefs drops its duplicate
copies too.

* test(node-host): write fake container-engine state atomically

checks-node-compact-large-3 failed twice on this branch in different
tests of node-worker-supervisor.container.test.ts, most recently with
"SyntaxError: Unexpected end of JSON input" raised from the fake engine's
load() while the supervisor ran `docker rm --force`.

The shim saved container state with a truncating fs.writeFileSync while
sibling shim invocations (rm, inspect, wait, ps) read the same path, so a
reader could observe the zero-length window and exit 1. Write to a
".pending" sibling and rename over the target instead; rename is atomic,
so readers always see a complete previous or next state. The ps handler
filters on the ".container.json" suffix, so pending files are ignored.

A concurrent write/read probe over the old pattern produced 2014 partial
reads in 212900; the same probe over write-then-rename produced 0 in
203468. Not reproducible end-to-end on macOS, where the truncation
window is far narrower than on the loaded Linux shard.