IsDefault (Enter-to-submit) is set in Viewer dialogs but not their Lite twins

[erikdarlingdata]

Noticed during #1847's keyboard sweep across both apps' dialogs and deliberately not touched there, because #1847's scope explicitly excluded adding or removing IsDefault (Enter-to-submit is a behavior change, not a keyboard affordance).

IsDefault="True" makes Enter press a button from anywhere in the window. Today it is set in the Darling Viewer and NOT in the equivalent Lite dialog, so the same dialog in the two SKUs answers Enter differently:

Dialog	Lite	Viewer
Add Multiple Servers - Add	no IsDefault	IsDefault="True"
Mute rule editor - Save	MuteRuleDialog, no IsDefault	MuteRuleEditDialog, IsDefault="True"
Add Server - Save	no IsDefault	no IsDefault (consistent, and deliberate per #1828)

Viewer-only dialogs with IsDefault and no Lite counterpart, listed for completeness: TagNameDialog (OK) and ManageTagsWindow (Close, which also carries IsCancel - Enter and Esc both close it).

The decision to make is which way to converge, and it is a real one rather than a rename:

• Add it to Lite and Enter submits the Add Multiple Servers and mute-rule dialogs there too - consistent, but Enter-from-a-half-filled-form is exactly what [BUG] Add Server dialog buttons disappear #1828 declined to introduce to Add Server.
• Remove it from the Viewer and both SKUs behave like Add Server does - consistent the other way, and a small behavior removal for existing Viewer users.

Either is fine; drifting is not, since the two front ends are meant to be the same product.