docs: retire the stray top-level web-ui.md, fold it into web-ui/README.md

docs/web-ui.md duplicated the web-ui/ directory name at the top level --
the only such collision in docs/ (every other subsystem has just a
directory, no sibling <dir>.md file). That's exactly why it rendered
outside the directory structure in the docs site nav (mara's report,
hyperhive#4054): the site build walks docs/ generically with no
special-casing, so a loose top-level file next to a same-named
directory shows up as its own flat top-level entry instead of nesting
under that directory's section.

web-ui.md's own first paragraph already said as much -- 'This doc has
been split for readability... start at web-ui/README.md instead.' It
was a leftover pointer from before the split, not a page carrying
unique content on its own merit.

Folded its two sections web-ui/README.md didn't already have (the
swarm-ui design-guide link, and the task-oriented 'reading paths'
quick-lookup list) into web-ui/README.md's existing 'More depth'
section, then deleted the stray file and repointed every real
reference at it: 3 in-tree doc cross-links, 3 doc prose mentions
(retargeted to the more specific dashboard.md/shape.md sub-page each
one was actually about), and ~28 frontend source comments
(dashboard/agent/shared packages) that cited it as
'docs/web-ui.md::<heading>' for implementation context -- retargeted
each to whichever of dashboard.md/shape.md/agent.md actually carries
that heading now, verified against each file's real heading list
rather than guessed.

Verified via scripts/check-doc-refs.sh (the same lint CI runs): 0 dead
pointers, both before write (confirming the tree was clean beforehand)
and after (confirming nothing broke).

frontend/packages/dashboard/src/common.js

@@ -68,7 +68,7 @@ export const form = (
 // ─── shared-worker SSE pipe ─────────────────────────────────────────────
 // Returns an EventSource-shaped facade backed by a SharedWorker that
 // holds one upstream `new EventSource(url)` and fans events out to
-// every connected tab. See docs/web-ui.md (SSE multiplexing paragraph)
+// every connected tab. See docs/web-ui/shape.md (SSE multiplexing paragraph)
 // for the design + Firefox throttling motivation; graceful fallback to
 // direct EventSource on environments without SharedWorker.
 //
@@ -105,7 +105,7 @@ function getSharedPort() {
 // SharedWorker death detection: pings from the worker bump the
 // activity clock; a visibility-gated watchdog polls and re-subscribes
 // on a fresh port if the page has been silent past the threshold.
-// See docs/web-ui.md (Worker-death self-heal paragraph) for the
+// See docs/web-ui/shape.md (Worker-death self-heal paragraph) for the
 // timing rationale + Firefox reclaim symptom.
 const WORKER_DEAD_THRESHOLD_MS = 90_000;
 const WORKER_WATCHDOG_INTERVAL_MS = 15_000;