feat(ui): make Fleet, Settings, and the dashboard table usable on mobile (#1331)

* feat(ui): make Fleet, Settings, and the dashboard table usable on mobile

Tier 2 of the mobile pass, all gated below the md breakpoint so desktop
renders identically:
- Fleet: the tab strip scrolls horizontally and the action row (Check
  Updates / Refresh / Add Node) wraps instead of clipping.
- Settings: below md the nav rail is a full-screen list; choosing a section
  pushes it full-screen with a back affordance (master/detail), matching the
  stack flow. Desktop keeps the two-pane layout.
- Dashboard: the fixed stack-health table scrolls horizontally on a phone.

Adds a Playwright desktop visual-regression spec (1280/1440/1920) as a
zero-desktop-change gate; its environment-specific snapshots are gitignored.

* test(ui): harden the desktop visual-regression gate

Mask only the live sidebar ticker and notification count instead of the whole
sidebar / top-bar shell, so the gate now proves that shell unchanged too.
Lower the pixel budget to 1200 (the only residual churn is in-content live
stats); run against seeded / frozen data in CI for a zero-tolerance gate. Adds
a data-testid to the activity ticker so it can be masked precisely.

frontend/src/components/dashboard/StackHealthTable.tsx

@@ -15,7 +15,10 @@ interface StackHealthTableProps {
 const PAGE_SIZE = 8;
 const WARN = 80;
 const CRIT = 90;
-const GRID_TEMPLATE = 'grid-cols-[14px_minmax(0,1fr)_minmax(0,120px)_52px_52px_72px_110px_16px]';
+// Shared by the header and data rows so their columns stay aligned. The
+// `max-md:min-w` keeps both at the same width below md, where the card scrolls
+// horizontally; desktop is unaffected by the `max-md:` prefix.
+const GRID_TEMPLATE = 'grid-cols-[14px_minmax(0,1fr)_minmax(0,120px)_52px_52px_72px_110px_16px] max-md:min-w-[600px]';
 
 const formatMemory = (mb: number): string => {
   if (mb >= 1024) return `${(mb / 1024).toFixed(1)} GB`;
@@ -148,7 +151,7 @@ export function StackHealthTable({
   }
 
   return (
-    <div className="rounded-lg border border-card-border border-t-card-border-top bg-card shadow-card-bevel">
+    <div className="rounded-lg border border-card-border border-t-card-border-top bg-card shadow-card-bevel max-md:overflow-x-auto">
       <div className="flex items-center justify-between gap-4 px-5 py-4">
         <div className="flex items-baseline gap-3">
           <h2 className="font-display italic text-xl leading-none tracking-tight text-stat-value">