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/__tests__/folderMove.test.ts

@@ -1,5 +1,5 @@
 import { describe, expect, it } from "vitest";
-import { canDropFolder, descendantIds, movable } from "../folderMove";
+import { canDropFolder, descendantIds, folderColor, movable } from "../folderMove";
 import type { Id, Mailbox } from "@/jmap/types";
 
 const mb = (id: string, name: string, parentId: string | null, role: Mailbox["role"] = null): Mailbox =>
@@ -67,3 +67,17 @@ describe("canDropFolder", () => {
     expect(canDropFolder(tree, "gone", "work")).toBe(false);
   });
 });
+
+describe("folderColor", () => {
+  it("returns the colour chosen for that folder, and null for the rest", () => {
+    const colors = { work: "#7c3aed" };
+    expect(folderColor(colors, "work")).toBe("#7c3aed");
+    expect(folderColor(colors, "news")).toBeNull();
+    expect(folderColor({}, "work")).toBeNull();
+  });
+
+  it("is keyed by id, so a renamed folder keeps its colour", () => {
+    // The id is stable across a rename; the name and path are not.
+    expect(folderColor({ mb1: "#0f766e" }, "mb1")).toBe("#0f766e");
+  });
+});