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/host_tile_test.go

@@ -62,10 +62,11 @@ func TestHostTile_OverlayLink(t *testing.T) {
 		t.Fatalf("tile missing tile-link class: %s", html)
 	}
 	// Dropped content that used to live on the tile — confirm it has
-	// actually moved off so the slim-down is real.
+	// actually moved off so the slim-down is real. tile-status is
+	// intentionally re-added as a minimal status badge (issue #11).
 	for _, dropped := range []string{
 		`tile-meta`, `tile-log`, `tile-actions`, `tile-hold`,
-		`tile-primary-action`, `tile-status`, `tile-start-form`,
+		`tile-primary-action`, `tile-start-form`,
 		`tile-nd-toggle`, `tile-cancel-form`,
 		`/hosts/42/start`, `/hosts/42/cancel`,
 		`Start vetting`, `Non-destructive`, `Cancel run`, `View report`,
@@ -129,10 +130,10 @@ func TestTileStatusCancelledFromHold(t *testing.T) {
 			wantMood:   "fail",
 		},
 		{
-			name:       "mid-stage cancel stays plain cancelled",
+			name:       "mid-stage cancel gets cancelled mood",
 			run:        &model.Run{State: model.StateCancelled},
 			wantStatus: "Cancelled",
-			wantMood:   "idle",
+			wantMood:   "cancelled",
 		},
 		{
 			name:       "failed-holding itself still reads as FailedHolding",