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/flow.css

@@ -1,5 +1,5 @@
 /* ─── /flow.html — full-page chat ─────────────────────────────────
-   See docs/web-ui.md::FL0W page for the page-vs-pane rationale.
+   See docs/web-ui/dashboard.md::FL0W page for the page-vs-pane rationale.
    Shape mirrors the per-agent live page (frosted-glass header +
    composer, full-viewport terminal). */
 
@@ -68,7 +68,7 @@ body.flow-shell .flow-main-slim {
 /* Tail pill (↓ N new): bottom offset clears the floating composer.
    Pill is anchored on .flow-main (not .terminal-wrap) so the
    backdrop-filter stacking context doesn't trap its z-index — see
-   docs/web-ui.md::Per-agent page (Terminal-wrap) for the same
+   docs/web-ui/agent.md::Per-agent page (Terminal-wrap) for the same
    gotcha on the agent page. */
 .flow-main .tail-pill {
   bottom: calc(var(--flow-composer-h) + 0.6em);