On the Start page there is no drawing to act on, but ribbon panel
command icons still dispatched into the empty welcome tab. Guard
on_ribbon_tool_click: when the active tab is the Start page, close any
open flyout, show a "use New or Open first" hint, and do nothing. The
quick-access New / Open / Save buttons use a separate path and stay
available, as do the welcome page's own actions.
The icons are not yet visually greyed out (that needs an enabled flag
threaded through the ribbon render) — this stops them from running.
Refs #299
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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