refactor(tui): decompose god files into single-concern modules

tui.go (1032→164) and tab_sites.go (993→482) violated "small functions"
and "testable in isolation" standards. Extracted 6 new files by concern:

- format.go: pure formatting functions (fmtLatency, fmtUptime, etc.)
- sparkline.go: sparkline rendering (latency, heartbeat, group)
- update.go: Update method decomposed into 15 named handlers
- view_dashboard.go: View, dashboard composition, tab bar, footer
- view_detail.go: site detail panel
- data.go: data refresh with extracted sortSitesForDisplay/filterSites

Added 17 unit tests for the newly-testable pure functions covering
format, sparkline, sort ordering, and filter logic. No behavioral
changes — strict move-and-extract refactor.

internal/tui/data_test.go

@@ -0,0 +1,95 @@
+package tui
+
+import (
+	"testing"
+
+	"gitea.lerkolabs.com/lerkolabs/uptop/internal/models"
+)
+
+func TestSortSitesForDisplay_GroupsFirst(t *testing.T) {
+	sites := []models.Site{
+		{ID: 3, Name: "ungrouped", Type: "http", Status: "UP"},
+		{ID: 1, Name: "group-a", Type: "group", Status: "UP"},
+		{ID: 2, Name: "child", Type: "http", Status: "UP", ParentID: 1},
+	}
+	result := sortSitesForDisplay(sites, nil)
+	if len(result) != 3 {
+		t.Fatalf("expected 3 sites, got %d", len(result))
+	}
+	if result[0].Name != "group-a" {
+		t.Errorf("first should be group, got %s", result[0].Name)
+	}
+	if result[1].Name != "child" {
+		t.Errorf("second should be child, got %s", result[1].Name)
+	}
+	if result[2].Name != "ungrouped" {
+		t.Errorf("third should be ungrouped, got %s", result[2].Name)
+	}
+}
+
+func TestSortSitesForDisplay_CollapsedHidesChildren(t *testing.T) {
+	sites := []models.Site{
+		{ID: 1, Name: "group-a", Type: "group", Status: "UP"},
+		{ID: 2, Name: "child-1", Type: "http", Status: "UP", ParentID: 1},
+		{ID: 3, Name: "child-2", Type: "http", Status: "UP", ParentID: 1},
+	}
+	collapsed := map[int]bool{1: true}
+	result := sortSitesForDisplay(sites, collapsed)
+	if len(result) != 1 {
+		t.Fatalf("collapsed group should hide children, got %d items", len(result))
+	}
+	if result[0].Name != "group-a" {
+		t.Errorf("only group should remain, got %s", result[0].Name)
+	}
+}
+
+func TestSortSitesForDisplay_StatusOrdering(t *testing.T) {
+	sites := []models.Site{
+		{ID: 1, Name: "up-site", Type: "http", Status: "UP"},
+		{ID: 2, Name: "down-site", Type: "http", Status: "DOWN"},
+		{ID: 3, Name: "late-site", Type: "http", Status: "LATE"},
+	}
+	result := sortSitesForDisplay(sites, nil)
+	if result[0].Status != "DOWN" {
+		t.Errorf("DOWN should sort first, got %s", result[0].Status)
+	}
+	if result[1].Status != "LATE" {
+		t.Errorf("LATE should sort second, got %s", result[1].Status)
+	}
+	if result[2].Status != "UP" {
+		t.Errorf("UP should sort third, got %s", result[2].Status)
+	}
+}
+
+func TestFilterSites(t *testing.T) {
+	sites := []models.Site{
+		{Name: "Production API"},
+		{Name: "Staging API"},
+		{Name: "Database"},
+	}
+
+	tests := []struct {
+		needle string
+		want   int
+	}{
+		{"api", 2},
+		{"API", 2},
+		{"database", 1},
+		{"nonexistent", 0},
+		{"", 3},
+	}
+	for _, tt := range tests {
+		got := filterSites(sites, tt.needle)
+		if len(got) != tt.want {
+			t.Errorf("filterSites(%q) returned %d, want %d", tt.needle, len(got), tt.want)
+		}
+	}
+}
+
+func TestFilterSites_EmptyNeedle(t *testing.T) {
+	sites := []models.Site{{Name: "a"}, {Name: "b"}}
+	got := filterSites(sites, "")
+	if len(got) != 2 {
+		t.Errorf("empty needle should return all, got %d", len(got))
+	}
+}