style(config): give Codex the same tab bar as Claude Code, drop the Cursor tab

The Codex tab strip wrapped onto a second row while Claude Code's stayed
one scrollable row, so two panels that are meant to be twins read as
different components. Both now render the same ConfigTabBar — the
scroller lifted out of CcConfig, with the fade edges and chevrons that
only appear when there is somewhere to scroll. The deferral note in
CodexConfig that predicted this is gone with it.

The bar also carries role="tablist"/"tab" and aria-selected now: Codex
had them, Claude Code did not, and one of the two had to win.

Cursor's placeholder tab goes away with the machinery that served only
it — the optional Panel, the "soon" badge, the coming-soon card, and
their four i18n keys.

Element.scrollBy and friends move to test-setup: jsdom has none of them,
and the tab bar now calls them from both pages.

docs/CLI.md

@@ -110,7 +110,7 @@ The CLI talks to the local dashboard server — **API-backed commands require it
          /\    \                  /\    \                  /\    \                  /\    \
         /::\    \                /::\    \                /::\    \                /::\____\
         …  (AgentHub word-mark)  …
-  AgentHub · interactive shell · v1.4.6   ● 127.0.0.1:4820
+  AgentHub · interactive shell · v1.5.0   ● 127.0.0.1:4820
   Type commands without the 'agenthub' prefix — e.g. sessions --limit 5
   help all commands · help <cmd> details · Tab completes · ↑/↓ history · exit to quit