swarm-ui: neon glow on panel titles + the active nav tab

Ports the subtle text-shadow glow @hive/agent's and the dashboard's own
headline rule already use (both independently converged on
`text-shadow: 0 0 Npx color-mix(color, transparent)`), not their whole
uppercase+letter-spacing headline typography -- that's those UIs' own
look, swarm-ui never adopted it and this issue only asked for the glow.

- Panel titles: `currentColor`-based, since a panel title has no one
  fixed accent (some are dynamic, e.g. an agent's own name) -- a fixed
  --purple glow on plain --fg text would read as a mismatched halo.
- Active nav tab: each item's own `accent` (NAV_ITEMS), the same value
  its underline already uses, so the glow and the underline read as
  one accent rather than two that happen to agree.

frontend/packages/swarm-ui/src/ui/panel/Panel.css

@@ -14,6 +14,16 @@
   margin: 0;
   font-size: 1em;
   font-weight: 600;
+  /* Same subtle neon-glow recipe agent.css/dashboard's own headline
+     rules use (`text-shadow: 0 0 Npx color-mix(color, transparent)`) —
+     `currentColor`, not a fixed `--purple` like those two, since panel
+     titles don't carry one fixed accent (some are dynamic, e.g. an
+     agent's own name) and forcing one on plain `--fg` text would read
+     as a stray mismatched halo rather than a glow. Deliberately not
+     porting those UIs' uppercase+letter-spacing headline treatment
+     alongside it — that's their own typographic language, not part of
+     "the glow effect" itself. */
+  text-shadow: 0 0 8px color-mix(in srgb, currentColor 40%, transparent);
 }
 .ui-panel-icon {
   font-size: 1.2em;