Add an option to show issue/PR number as a dedicated column

Right now the issue/PR number only shows up as a #123 prefix on the title, and only when theme.ui.table.compact is enabled. It'd be great to be able to display the number without switching the whole table to compact mode.

Two possible approaches:

• A dedicated number column in the layout config (like repo, title, assignees, etc.), with the usual width/hidden options.
• Or decouple the #number title prefix from compact so it can be toggled on its own.

The number is really handy when cross-referencing with the web UI, CI, or branch names, so having it visible regardless of the compact setting would be a nice quality-of-life improvement.