swarm-ui: don't break at a narrow viewport

Closes #3447.

- Shell.css: .shell-header and .shell-nav wrap onto a second line
  below a breakpoint instead of forcing horizontal overflow — no
  hamburger/collapse menu, per mara's own 'don't break, don't
  over-invest' scoping.
- LinksMenu.css: bumped .links-menu-button and .links-menu-item to the
  same 2.75em (~44px, WCAG 2.5.5) touch-target floor the shared ui/
  kit's form controls and buttons already carry — this trigger is
  bespoke, not built from that kit, but the floor applies regardless.
- Table.tsx/.css: wrapped the table in a .ui-table-scroll container
  (overflow-x: auto) so a wide table scrolls within its own box on a
  narrow viewport instead of forcing the whole page to overflow
  horizontally — not named in the issue's two concrete spots, but the
  same 'should not break' failure mode, and fixing it at the shared
  Table primitive covers every future caller.

.create-agent-input's fixed width -> max-width was already fixed as
part of #3448's form kit.

Verified: tsc clean, build succeeds, screenshotted at 320px (hives
nav wraps, table scrolls internally, page itself doesn't overflow)
and 480px (create-agent form fields fit).

frontend/packages/swarm-ui/src/shell/LinksMenu.css

@@ -1,6 +1,10 @@
 /* <LinksMenu> — a single header icon-button + popover. Kept visually
    quiet (no border/fill until interacted with) so it reads as chrome,
-   not another nav item. */
+   not another nav item. Button is sized to the same touch-target floor
+   as the shared `ui/` kit (2.75em ≈ 44px, WCAG 2.5.5) even though it
+   isn't built from that kit — a bespoke icon trigger, not a form
+   control, but the floor applies regardless of which component drew
+   it. */
 .links-menu {
   position: relative;
   margin-left: auto;
@@ -9,8 +13,8 @@
   display: flex;
   align-items: center;
   justify-content: center;
-  width: 2em;
-  height: 2em;
+  width: 2.75em;
+  height: 2.75em;
   padding: 0;
   border: 1px solid transparent;
   border-radius: 0.4em;
@@ -43,6 +47,7 @@
   display: flex;
   align-items: center;
   gap: 0.5em;
+  min-height: 2.75em;
   padding: 0.4em 0.6em;
   border-radius: 0.35em;
   color: var(--fg);