fix(composer): larger fonts and Enter-to-advance in new-workspace dialog

[AmethystLiang]

Summary

• Bump field fonts in the Cmd+N New Workspace dialog so labels/inputs aren't cramped (labels text-xs, inputs text-sm h-9, incl. Note and Setup script labels).
• Enter on Workspace Name now moves focus to the Agent combobox instead of doing nothing.
• Enter on the Agent combobox trigger (closed) submits the form via a new optional onTriggerEnter prop, guarded by createDisabled.
• Small pt-3 on the fields container so the form sits slightly lower under the dialog header.

Test plan

• Open Cmd+N, verify labels/inputs are legible and aligned.
• Type a workspace name, press Enter → focus lands on Agent combobox trigger.
• With the Agent combobox closed, press Enter → workspace is created.
• Cmd/Ctrl+Enter submit still works globally inside the dialog.
• Agent combobox still opens on ArrowDown/ArrowUp and seeds query on printable chars.