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/keys.go

@@ -17,6 +17,7 @@ const (
 	ActionToggleMenuPane
 	ActionToggleListPane
 	ActionToggleDetailPane
+	ActionFilter
 	ActionNextPane
 	ActionPrevPane
 	ActionRefresh
@@ -38,6 +39,8 @@ func KeyAction(key string, pane model.FocusPane) Action {
 		}
 	case "r":
 		return ActionRefresh
+	case "f":
+		return ActionFilter
 	case "up", "k":
 		return ActionMoveUp
 	case "down", "j":