v0.6.21

feat: menu bar (File/Connection/View/Help) with shortcuts; trim footer

Replace the ad-hoc title-bar buttons with a proper menu bar, as sketched:
File (Record, Quit), Connection (Connect…), View (Debug, trace/aggregate,
Settings…), Help (About, Keyboard help). Clicking a title opens a dropdown
positioned under it, listing each action with its keyboard shortcut; selecting
runs the action after the menu closes (so a dialog it opens isn't popped with
the menu). Keyboard shortcuts are unchanged.

The footer now carries just navigation + layout hints; the action keys live in
the menu. Dialog-open tests now route through the menu items.

Bumps to 0.6.21.