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/agent/src/agent.css

@@ -16,7 +16,7 @@
 
 /* ─── full-screen layout overrides ─────────────────────────────────
    The agent page mounts a full-viewport terminal under a fixed
-   frosted-glass header + composer pair. See docs/web-ui.md::Per-agent
+   frosted-glass header + composer pair. See docs/web-ui/agent.md::Per-agent
    page for the layer / pill / side-panel structure; rules below
    override the in-page defaults from @hive/shared. */
 
@@ -175,7 +175,7 @@ h3 {
 }
 .agent-icon {
   /* Square identity anchor — explicit 5em sizing + align-self.
-     See docs/web-ui.md::Per-agent page (Agent icon) for the
+     See docs/web-ui/agent.md::Per-agent page (Agent icon) for the
      intrinsic-dim-pushes-parent-flex-open + sticks-to-top
      rationale. */
   width: 5em;
@@ -702,7 +702,7 @@ pre.diff {
    the composer (z-30) so the pill sits on the top layer instead of
    being clipped by the floating chrome. (Pill is anchored in
    `.agent-main` rather than `.terminal-wrap` — see
-   docs/web-ui.md::Per-agent page Terminal-wrap paragraph for the
+   docs/web-ui/agent.md::Per-agent page Terminal-wrap paragraph for the
    backdrop-filter stacking-context rationale.) */
 .agent-main .tail-pill {
   bottom: calc(var(--agent-composer-h) + 0.6em);

frontend/packages/agent/src/screen.html

@@ -61,7 +61,7 @@
         });
 
         // --- Fit-to-window toggle ---
-        // See docs/web-ui.md::Per-agent endpoints (GET /screen) for the
+        // See docs/web-ui/agent.md::Per-agent endpoints (GET /screen) for the
         // localStorage persistence + canvas-intrinsic-resolution +
         // pointer-rescale model.
         let fitMode = localStorage.getItem("screen-fit") !== "off";
@@ -105,7 +105,7 @@
         // Sends an RFB SetDesktopSize request so the VNC server (weston)
         // changes its actual output resolution to match the browser
         // viewport — sharper than fit-mode's CSS downscale. Gated on the
-        // server's ExtendedDesktopSize advert; see docs/web-ui.md::Per-agent
+        // server's ExtendedDesktopSize advert; see docs/web-ui/agent.md::Per-agent
         // endpoints (GET /screen).
         let extDesktopSupported = false;
         let screenId = 1; // captured from the server's ExtendedDesktopSize advert