feat: show every amount in one currency, chosen in the header, $ toggles it

A selector in the header and the palette's Currency group pick the
currency, the primary one by default; the choice persists between pages
and $ switches it on and off. Rates come from Firefly's exchange rates,
then from the owner's own transactions with a foreign amount, walking
through other currencies when there is no direct pair. Amount converts
with a ≈ and keeps the original in its title; Money sums several
currencies into one converted amount.

src/lib/dashboard/primary.svelte.ts

@@ -2,9 +2,9 @@ import { api } from "#lib/api/client.js";
 import { all } from "#lib/api/lists.js";
 import type { ExchangeRate, Page, TransactionGroup } from "#lib/api/types.js";
 import { formatMoney, toNumber } from "#lib/format.js";
+import { display } from "#lib/money/display.svelte.js";
 import { session } from "#lib/session.svelte.js";
 
-const KEY = "ffui.dashboard.in-primary";
 const OWN_SAMPLE = 100;
 const HOUR_MS = 3_600_000;
 
@@ -25,13 +25,12 @@ export function median(values: number[]): number {
 }
 
 class InPrimary {
-  on = $state(
-    typeof localStorage !== "undefined" && localStorage.getItem(KEY) === "1"
-  );
+  get on() {
+    return display.on && display.code === session.primary?.code;
+  }
 
   toggle() {
-    this.on = !this.on;
-    localStorage.setItem(KEY, this.on ? "1" : "0");
+    display.toggle();
   }
 }