refactor(tui): status icons, clean STATUS column, relative time

- STATUS column shows icon + clean state only (▲ UP, ▼ DOWN, ◆ LATE,
  ◆ STALE, ◇ PAUSED, ◼ MAINT, ○ PENDING). Error classification
  (DNS/TLS/TMO) removed from STATUS — stays in NAME inline hint.
- Detail panel Last Check shows relative time ("12s ago") instead of
  absolute timestamp.
- Extract shared fmtTimeAgo() to format.go, consolidate duplicate
  formatters in tab_alerts.go and tab_nodes.go.

internal/tui/format_test.go

@@ -55,32 +55,27 @@ func TestSiteOrder(t *testing.T) {
 	}
 }
 
-func TestFmtStatus_ErrorCategory(t *testing.T) {
+func TestFmtStatus(t *testing.T) {
 	tests := []struct {
 		status  string
 		paused  bool
 		inMaint bool
-		cat     ErrorCategory
 		wantSub string
 	}{
-		{"DOWN", false, false, ErrCatDNS, "DOWN:DNS"},
-		{"DOWN", false, false, ErrCatTLS, "DOWN:TLS"},
-		{"DOWN", false, false, ErrCatHTTP, "DOWN:HTTP"},
-		{"DOWN", false, false, ErrCatTCP, "DOWN:TCP"},
-		{"DOWN", false, false, ErrCatTimeout, "DOWN:TMO"},
-		{"DOWN", false, false, ErrCatICMP, "DOWN:ICMP"},
-		{"DOWN", false, false, ErrCatPrivate, "DOWN:PRIV"},
-		{"DOWN", false, false, ErrCatUnknown, "DOWN"},
-		{"UP", false, false, ErrCatUnknown, "UP"},
-		{"SSL EXP", false, false, ErrCatUnknown, "SSL EXP"},
-		{"DOWN", true, false, ErrCatDNS, "PAUSED"},
-		{"DOWN", false, true, ErrCatDNS, "MAINT"},
+		{"DOWN", false, false, "▼ DOWN"},
+		{"UP", false, false, "▲ UP"},
+		{"SSL EXP", false, false, "▼ SSL EXP"},
+		{"LATE", false, false, "◆ LATE"},
+		{"STALE", false, false, "◆ STALE"},
+		{"PENDING", false, false, "○ PENDING"},
+		{"DOWN", true, false, "◇ PAUSED"},
+		{"DOWN", false, true, "◼ MAINT"},
 	}
 	for _, tt := range tests {
-		got := fmtStatus(tt.status, tt.paused, tt.inMaint, tt.cat)
+		got := fmtStatus(tt.status, tt.paused, tt.inMaint)
 		if !containsPlain(got, tt.wantSub) {
-			t.Errorf("fmtStatus(%q, paused=%v, maint=%v, %q): %q missing %q",
-				tt.status, tt.paused, tt.inMaint, tt.cat, got, tt.wantSub)
+			t.Errorf("fmtStatus(%q, paused=%v, maint=%v): %q missing %q",
+				tt.status, tt.paused, tt.inMaint, got, tt.wantSub)
 		}
 	}
 }