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/store/settings.ts

@@ -62,6 +62,11 @@ export interface Settings {
   fontSize: "small" | "medium" | "large";
   templates: Template[];
   labels: Array<{ keyword: string; name: string; color: string }>;
+  /**
+   * Folder colours, by mailbox id. Local to this browser, like every other
+   * colour here: JMAP has nowhere on a Mailbox to keep one.
+   */
+  folderColors: Record<string, string>;
   sidebarCollapsed: boolean;
   showHiddenFolders: boolean;
   trustedImageSenders: string[];
@@ -116,6 +121,7 @@ export const DEFAULT_SETTINGS: Settings = {
   fontSize: "medium",
   templates: [],
   labels: [],
+  folderColors: {},
   sidebarCollapsed: false,
   showHiddenFolders: false,
   trustedImageSenders: [],