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_resize_test.go

@@ -0,0 +1,41 @@
+package ui
+
+import "testing"
+
+func TestDividerAtBetweenMenuAndList(t *testing.T) {
+	pw := PaneWidths{
+		Menu: 30, List: 50, Detail: 40,
+		Visibility: PaneVisibility{Menu: true, List: true, Detail: true},
+	}
+	if got := DividerAt(30, pw); got != 0 {
+		t.Fatalf("divider at 30=%d want 0", got)
+	}
+	if got := DividerAt(80, pw); got != 1 {
+		t.Fatalf("divider at 80=%d want 1", got)
+	}
+}
+
+func TestDragPaneDividerRespectsMinimum(t *testing.T) {
+	pw := PaneWidths{
+		Menu: 40, List: 40, Detail: 40,
+		Visibility: PaneVisibility{Menu: true, List: true, Detail: true},
+	}
+	next, ok := DragPaneDivider(pw, 0, -30)
+	if ok {
+		t.Fatalf("expected resize blocked by minimum, got %+v", next)
+	}
+	next, ok = DragPaneDivider(pw, 0, 5)
+	if !ok || next.Menu != 45 || next.List != 35 {
+		t.Fatalf("resize failed: ok=%v %+v", ok, next)
+	}
+}
+
+func TestFitPaneWidthsScalesToTotal(t *testing.T) {
+	vis := PaneVisibility{Menu: true, List: true, Detail: true}
+	sizes := PaneWidths{Menu: 20, List: 30, Detail: 50, Visibility: vis}
+	out := FitPaneWidths(120, vis, sizes)
+	sum := out.Menu + out.List + out.Detail
+	if sum != 120 {
+		t.Fatalf("sum=%d want 120", sum)
+	}
+}