Add single relay column

[alltheseas]

-see only notes from a single relay
-write only to single relay in this column

[alltheseas]

@robagreda I see there is a add "all relays" column option in the design: https://www.figma.com/design/aONNEWCxHlckZJq0lHyxrN/Notedeck?node-id=2228-18843&t=lP95ro7nco018edr-1

What does a "add single relay" button look like, and what does the single relay selector look like? Can there be:

1. an option to select a single relay from all relays in my relay list, and
2. an option to paste or type in a new relay that I do not have in my relay list?

For instance, I want to have a column for the cats only relay wss://catstrr.swarmstr.com

[alltheseas]

Unlike on iOS Damus where everything is written to the customer's relay list, on the single relay column there should be a signifier that communicates
-only notes written to relay X are displayed
-if you write on this column, notes only posted to relay X

[alltheseas]

suggested product requirement: display relay icon on column

[alltheseas]

recommend adding for beta milestone @jb55 https://github.com/damus-io/notedeck/milestone/2