fix(redesign): a11y, mobile, perf polish for production push

Comprehensive pre-production sweep across the Aurora redesign — drives
svelte-check to 0 errors / 0 warnings (was 61) without changing visual
intent. Highlights:

- Mobile: hero title shrinks at 480px, signal-list stacks timestamp
  under sentence below 640px, sidebar icon buttons bumped to 40x40
- Light theme: muted-foreground darkened to #3a3560 to clear WCAG AA
  on glass surfaces and the modal close button
- Perf: topbar backdrop-filter 28→14px, mobile-more sheet 28→12px to
  cut concurrent blur layers on mid-tier mobile
- a11y: prefers-reduced-motion mute for aurora drift / pulses /
  shimmer / stagger; aria-label on every icon-only button;
  aria-describedby on Hint; combobox/listbox/aria-activedescendant on
  SearchPalette; modal dialog tabindex; 47 label-without-control
  warnings across 14 form pages cleaned up via for=/id= or label→div
- Dashboard derived state split into topology- vs status-bound layers
  so polling no longer re-runs the full provider/wires computation
- Mobile bottom nav derived from baseNavEntries by key lookup so
  adding a top-level nav entry keeps the two trees in sync
- Bug: template-configs page now respects the global provider filter
  for both the count meter and the type pill (was reading the
  unfiltered cache)
- Misc: portal EventChart tooltip and switch its swatches to Aurora
  tokens; CollapsibleSlot warning state uses warning-fg/-bg tokens
  instead of #d97706; Hint z-index 99999→9999; element refs across
  Modal/EntitySelect/MultiEntitySelect/SearchPalette/IconGridSelect/
  Hint/targets converted to \$state for reactivity; 4 dead
  .topbar-cta selectors removed

frontend/src/routes/providers/+page.svelte

@@ -202,7 +202,7 @@
 		<ErrorBanner message={error} />
 		<form onsubmit={save} class="space-y-3">
 			<div>
-				<label class="block text-sm font-medium mb-1">{t('providers.type')}</label>
+				<div class="block text-sm font-medium mb-1">{t('providers.type')}</div>
 				{#if !editing}
 					<IconGridSelect items={providerTypeItems()} bind:value={form.type} columns={2} />
 				{:else}
@@ -247,7 +247,7 @@
 			{/each}
 			{#if descriptor?.webhookUrlPattern && editing}
 			<div class="bg-[var(--color-muted)] rounded-md p-3">
-				<label class="block text-sm font-medium mb-1">{t('providers.webhookUrl')}</label>
+				<div class="block text-sm font-medium mb-1">{t('providers.webhookUrl')}</div>
 				<code class="text-xs select-all break-all">{descriptor.webhookUrlPattern.replace('{token}', providers.find(p => p.id === editing)?.webhook_token ?? '')}</code>
 				<p class="text-xs text-[var(--color-muted-foreground)] mt-1">{t('providers.webhookUrlHint')}</p>
 			</div>