fix(frontend): responsive layout across all dashboard pages

- SectionHeader: action (filters/legends) now wraps below the title on narrow
  screens instead of overflowing beside it (flex-wrap, gap-2 sm:gap-3).
- GuildChannelPicker: selects go full-width and stack on mobile (w-full
  sm:w-44 / sm:w-52) instead of fixed widths that exceeded a 375px viewport.
- Messages search: w-full sm:w-64 so it doesn't crowd the picker on mobile.
- TopBar: tighter padding (px-4 sm:px-5), smaller title on mobile, connection
  status uses compact (dot only) on mobile, ambient pill hidden < sm.
- Shell main + dashboard channel label: responsive padding / shrink-0 widths.

Verified tsc --noEmit + next build clean; targets breakpoints 375/768/1024/1440.

services/frontend/src/app/(dashboard)/dashboard/view.tsx

@@ -155,7 +155,7 @@ export function DashboardView({
                 100;
               return (
                 <div key={c.channel_id} className="flex items-center gap-3">
-                  <span className="w-40 truncate text-sm text-ink-soft">
+                  <span className="w-28 shrink-0 truncate text-sm text-ink-soft sm:w-40">
                     {c.channel_name ?? c.channel_id.slice(0, 8)}
                   </span>
                   <div className="h-2 flex-1 overflow-hidden rounded-full bg-white/8">