Give a folder a colour from its right-click menu

Right-click a folder and pick one of the twelve colours the calendar
already uses, or clear it again. The colour tints the folder's icon; the
label keeps the sidebar's own contrast, which a dozen arbitrary colours
would not reliably give it.

Kept by mailbox id rather than by name, so a folder renamed or dragged
somewhere else keeps its colour. Stored in settings, which live in this
browser -- JMAP has nowhere on a Mailbox to put a colour, and every other
colour in the app, labels and event categories included, already works
this way. Worth knowing it does not follow you to another device.

The cascade needed care: .nav-item svg sets the colour on the icon
itself, so a colour inherited from a wrapper does nothing. Checking
getComputedStyle on the wrapper said the icon was purple while the pixels
stayed grey; the rule now targets the svg, and the check now reads the
pixels.

web/src/lib/folderMove.ts

@@ -45,3 +45,8 @@ export function canDropFolder(mailboxes: Record<Id, Mailbox>, draggedId: Id, tar
   if (!mailboxes[targetId]) return false;
   return !descendantIds(mailboxes, draggedId).has(targetId);
 }
+
+/** The colour chosen for a folder, if any. Ids are used, so a rename keeps it. */
+export function folderColor(colors: Record<string, string>, id: Id): string | null {
+  return colors[id] ?? null;
+}