feat(ui): 15-point UX overhaul — affordances, feedback, and navigation

Address friction points identified in a full interface audit:
- Re-add status badge to dashboard tiles so run state is visible at a glance
- Add active nav indicator and SSE connection health monitor (live/stale)
- Show manual registration form by default instead of hiding behind <details>
- Add copy-to-clipboard buttons on SSH hold command and quick-register one-liner
- Replace tooltip-only profile descriptions with inline visible text
- Clarify non-destructive toggle with explicit stage impact description
- Replace disabled "Start vetting" button with actionable offline guidance
- Swap browser confirm() dialogs for styled inline confirmations
- Add colored badge to spec diffs summary visible when collapsed
- Add distinct "cancelled" mood for cancelled runs (vs idle)
- Add match count to log search and aria-label for accessibility
- Add styled 404 page rendered inside the app shell

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/web/templates/not_found.templ

@@ -0,0 +1,11 @@
+package templates
+
+templ NotFound() {
+	@Layout("Not found") {
+		<section class="not-found">
+			<h1>Page not found</h1>
+			<p>The host or run you're looking for doesn't exist or has been deleted.</p>
+			<a class="button" href="/">Back to dashboard</a>
+		</section>
+	}
+}