Partner commanders: Partner, Partner—<variant>, Partner with, Background, Doctor

A Commander/Brawl deck can now have two commanders when their abilities
allow it. Partner abilities are parsed from oracle text (Scryfall's keyword
list files every "Partner—Survivors"-style variant under plain "Partner"),
and a second search box offers only cards that can pair with the commander.
The legality check accepts a valid pair and checks color identity against
both; suggestions and decklist import/export handle both commanders.

Closes #4

src/styles.css

@@ -2587,6 +2587,10 @@ tbody tr:hover {
 }
 .deck-top-commander {
   grid-area: commander;
+  /* One hero per commander: a partner pair stacks. */
+  display: flex;
+  flex-direction: column;
+  gap: 16px;
 }
 .deck-top-meta {
   grid-area: meta;