feat(tui): add help overlay shortcuts (F1/?) and update help UI and status messages

- Introduced a new “HELP & QUICK COMMANDS” section with bold header and shortcuts for toggling the help overlay and opening the files panel.
- Updated command help text to “Open the help overlay”.
- Extended onboarding and tutorial status lines to display the help shortcut.
- Modified help command handling to set the status to “Help” and clear errors.

crates/owlen-tui/src/commands/mod.rs

@@ -90,7 +90,7 @@ const COMMANDS: &[CommandSpec] = &[
     },
     CommandSpec {
         keyword: "help",
-        description: "Show help documentation",
+        description: "Open the help overlay",
     },
     CommandSpec {
         keyword: "h",