fix(hig): give Copy with Headers a default keyboard shortcut (#1490)

[datlechin]

Part of #1490 (keyboard, focus, accessibility). Menus & shortcuts surface.

ShortcutAction.copyWithHeaders exists and the Edit-menu item binds shortcut(for: .copyWithHeaders), but defaultShortcuts had no entry, so the menu item showed with no key and the action was keyboard-unreachable by default. This adds Cmd+Option+C (verified free; the other c combos are Cmd+C, Cmd+Shift+C, Cmd+Ctrl+C). Rebindable in Settings, Keyboard. Docs updated.

The filter focus-return menu gaps from the audit are already merged (#1492). Deferred because they are UX-policy decisions rather than clear fixes:

• 'Find...' is disabled (not hidden/retitled) on table tabs where Cmd+F routes to Toggle Filters.
• Cmd+D is 'Save as Favorite', which collides with the NSAlert 'Don't Save' accelerator.
• New Table / ER Diagram / Server Dashboard and pagination have no menu-bar shortcuts (need new actions + key choices).

Lint clean, style gate clean.