feat(office): mail preview, infinite scroll, pane resize, and scrollbars

Polish the office TUI with proportional pane layout, drag resize, live
filtering, HTML mail preview, paginated mail lists, overflow scrollbars,
flattened navigation, and project save/status fixes; add UpdateProjectStatus.

Co-authored-by: Cursor <cursoragent@cursor.com>

cmd/office/ui/layout_test.go

@@ -13,6 +13,9 @@ func TestLayoutWidthsAllVisibleUsesFullWidth(t *testing.T) {
 	if sum != 120 {
 		t.Fatalf("sum=%d want 120", sum)
 	}
+	if pw.Menu != 12 || pw.List != 72 || pw.Detail != 36 {
+		t.Fatalf("menu=%d list=%d detail=%d want 12/72/36 (10/60/30)", pw.Menu, pw.List, pw.Detail)
+	}
 }
 
 func TestLayoutWidthsSinglePane(t *testing.T) {
@@ -30,6 +33,20 @@ func TestLayoutWidthsTwoPanes(t *testing.T) {
 	if pw.Menu+pw.Detail != 80 {
 		t.Fatalf("sum=%d want 80", pw.Menu+pw.Detail)
 	}
+	if pw.Menu != 20 || pw.Detail != 60 {
+		t.Fatalf("menu=%d detail=%d want 20/60 (10/30 of pair)", pw.Menu, pw.Detail)
+	}
+}
+
+func TestDetailPaneXRangeSkipsHidden(t *testing.T) {
+	pw := PaneWidths{
+		Menu: 20, List: 60, Detail: 0,
+		Visibility: PaneVisibility{Menu: true, List: true, Detail: false},
+	}
+	start, end := DetailPaneXRange(pw)
+	if start != 0 || end != 0 {
+		t.Fatalf("hidden detail should have empty range, got %d..%d", start, end)
+	}
 }
 
 func TestNextVisibleFocusSkipsHidden(t *testing.T) {