feat: every page follows the one-currency mode

Totals, info boxes, section headers, group subtotals, shares and the
per-currency charts on the dashboard, transactions, search, accounts,
merchants, statement, budgets, categories, tags, groups, piggy banks,
reports and the category, tag, budget and merchant pages sum into one
converted amount when the mode is on; charts split by currency merge
into one lane. Inputs, reconciliation and the Foreign column stay exact.

Money moved to or from a debt, loan or mortgage no longer counts as
income or spending in the transaction list boxes; it shows as From
debts and To debts. The running balance of an incoming foreign-currency
transfer shows in the account's currency.

src/lib/components/accounts/PeriodList.svelte

@@ -16,6 +16,7 @@
     type ViewRange,
   } from "#lib/dates.js";
   import { toNumber } from "#lib/format.js";
+  import { display } from "#lib/money/display.svelte.js";
   import { period, periodLabel } from "#lib/period.svelte.js";
 
   const COUNT = 12;
@@ -50,6 +51,7 @@
   }));
 
   interface Flow {
+    approx?: boolean;
     code: string;
     earned: number;
     spent: number;
@@ -76,12 +78,34 @@
     }
     return map;
   });
+  const merged = $derived.by(() => {
+    if (!display.active) {
+      return null;
+    }
+    const map = new Map<string, Flow[]>();
+    for (const [key, list] of byStart) {
+      const flow: Flow = { code: display.code, earned: 0, spent: 0 };
+      for (const f of list) {
+        const rate = display.rate(f.code);
+        if (rate === null) {
+          return null;
+        }
+        flow.earned += f.earned * rate;
+        flow.spent += f.spent * rate;
+        flow.approx ||= f.code !== display.code;
+      }
+      map.set(key, [flow]);
+    }
+    return map;
+  });
+  const shown = $derived(merged ?? byStart);
+  const main = $derived(merged ? display.code : code);
   const scale = $derived(
     Math.max(
       1,
-      ...[...byStart.values()].flatMap((list) =>
+      ...[...shown.values()].flatMap((list) =>
         list
-          .filter((f) => f.code === code)
+          .filter((f) => f.code === main)
           .map((f) => Math.max(f.earned, f.spent))
       )
     )
@@ -95,7 +119,7 @@
     class="-mr-px grid grid-cols-1 overflow-hidden @xs:grid-cols-2 @lg:grid-cols-3 @2xl:grid-cols-4 @4xl:grid-cols-6"
   >
     {#each blocks as block (block.start)}
-      {@const list = (byStart.get(block.start) ?? []).filter(
+      {@const list = (shown.get(block.start) ?? []).filter(
         (f) => f.earned || f.spent
       )}
       <li class="border-cladd-outline/60 border-r border-b">
@@ -117,17 +141,20 @@
           {/if}
           {#each list as flow (flow.code)}
             {@const fmt = money(currency(flow.code))}
+            {@const mark = flow.approx ? "≈" : ""}
             <span
               class="num flex flex-wrap justify-between gap-x-2 text-[12px]"
             >
               {#if flow.earned}
-                <span class="text-income">+{fmt(flow.earned)}</span>
+                <span class="text-income">{mark}+{fmt(flow.earned)}</span>
               {/if}
               {#if flow.spent}
-                <span class="ml-auto text-expense">−{fmt(flow.spent)}</span>
+                <span class="ml-auto text-expense"
+                  >{mark}−{fmt(flow.spent)}</span
+                >
               {/if}
             </span>
-            {#if flow.code === code}
+            {#if flow.code === main}
               <span aria-hidden="true" class="flex h-1 gap-0.5">
                 <span class="flex flex-1 justify-end">
                   <span