feat: reusable mint fee math with cached quotes, fee-aware Max in move funds

e2e/helpers/MintMocker.ts

@@ -52,6 +52,9 @@ interface MockTargetMintOptions {
   preimage: string;
   // value of the note a claim mints, in msat
   noteAmountMsat: number;
+  // optional payRequest metadata advertising a receive fee, e.g.
+  // '[["text/plain","Mint fees: 2000,0"]]' (flat 2000 msat, 0 ppm)
+  mintFeeMetadata?: string;
 }
 
 export class MintMocker {
@@ -127,7 +130,7 @@ export class MintMocker {
           maxSendable: 100_000_000_000,
           withdrawLink: `${origin}${NOTE_PATH}`,
           mintPubkey: options.mintPubkey,
-          metadata: '[]',
+          metadata: options.mintFeeMetadata ?? '[]',
         });
       },
     );

e2e/specs/move-funds.spec.ts

@@ -73,6 +73,31 @@ test.describe('Move funds', () => {
     await expect(page.getByRole('button', { name: 'Move now' })).toBeVisible();
   });
 
+  test('the Max helper discounts the target mint fee', async ({ page, mint }) => {
+    await fundSourceMint(page, mint);
+    // the target mint advertises a flat 2-sat receive fee (2000 msat, 0 ppm)
+    await mint.mockTargetMint(
+      {
+        mintPubkey: TARGET_PUBKEY,
+        invoice: INVOICE,
+        preimage: PREIMAGE,
+        noteAmountMsat: AMOUNT_MSAT,
+        mintFeeMetadata: '[["text/plain","Mint fees: 2000,0"]]',
+      },
+      MINT2_ORIGIN,
+    );
+
+    await page.goto('/#/settings/move');
+    await pickOption(page, 'From mint', 'mint.test - 50 sats available');
+    await pickOption(page, 'To mint', 'Another mint…');
+    await page.getByLabel('Target mint address').fill('@mint2.test');
+
+    // the fee quote lands (caption shown), then Max fills 50 - 2 = 48
+    await expect(page.getByText(/receive fee/)).toBeVisible();
+    await page.getByRole('button', { name: 'Max' }).click();
+    await expect(page.getByLabel('Amount')).toHaveValue('48');
+  });
+
   test('a two-mint transfer moves the balance', async ({ page, mint }) => {
     await fundSourceMint(page, mint);
     // the target mint: hands out the invoice, reports it settled with the