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/preview/entity_test.go

@@ -0,0 +1,58 @@
+package preview_test
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/eslider/go-onlyoffice/cmd/office/preview"
+)
+
+func TestMailMarkdownConvertsHTMLBody(t *testing.T) {
+	raw := map[string]any{
+		"subject":  "Weekly update",
+		"from":     `"Team" <team@example.com>`,
+		"htmlBody": `<p>Hello <strong>world</strong></p><p><a href="https://example.com">Link</a></p>`,
+	}
+	md := preview.MailMarkdown(raw)
+	if !strings.Contains(md, "Weekly update") {
+		t.Fatalf("missing subject:\n%s", md)
+	}
+	if !strings.Contains(md, "team@example.com") {
+		t.Fatalf("missing from:\n%s", md)
+	}
+	if strings.Contains(md, "<strong>") || strings.Contains(md, "<p>") {
+		t.Fatalf("expected HTML converted, got raw tags:\n%s", md)
+	}
+	if !strings.Contains(md, "world") {
+		t.Fatalf("missing body text:\n%s", md)
+	}
+}
+
+func TestMailMarkdownRendersWithColor(t *testing.T) {
+	md := preview.MailMarkdown(map[string]any{
+		"subject":  "Styled",
+		"from":     "a@b.com",
+		"htmlBody": `<h2>Heading</h2><p>Plain text</p>`,
+	})
+	out, err := preview.RenderMarkdown(md, 60)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if strings.TrimSpace(out) == "" {
+		t.Fatal("empty render output")
+	}
+	if !strings.Contains(out, "Heading") || !strings.Contains(out, "Plain text") {
+		t.Fatalf("missing rendered content:\n%s", out)
+	}
+}
+
+func TestMailMarkdownPlainBodyFallback(t *testing.T) {
+	md := preview.MailMarkdown(map[string]any{
+		"subject": "Hi",
+		"from":    "a@b.com",
+		"body":    "Just plain text",
+	})
+	if !strings.Contains(md, "Just plain text") {
+		t.Fatalf("missing plain body:\n%s", md)
+	}
+}