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/dashboard.html

@@ -294,7 +294,7 @@
          for per-target / whole-row cancel. Live updates via
          `schedules_changed` SSE; tab activation re-fetches as a
          safety net for disconnect windows.
-         See docs/web-ui.md::SCH3DUL3S tab. -->
+         See docs/web-ui/dashboard.md::SCH3DUL3S tab. -->
         <section
           class="tab-pane"
           id="tab-pane-schedules"
@@ -354,7 +354,7 @@
 
     <!-- Selection action bar. Sticky-bottom strip that slides into
        view when one or more agent cards is selected (click the icon
-       to toggle). See docs/web-ui.md::Selection bar for the bulk
+       to toggle). See docs/web-ui/dashboard.md::Selection bar for the bulk
        action gating + clear semantics. -->
     <div
       id="selection-bar"