fix: show balances and fees below 0.000001 ETH as nonzero on the send screens (closes #343)

The stored ETH balance and the send-confirm screen's fee were each cut to six
decimal places, so a value below 0.000001 read as 0.0. The ETH balance is now
stored exactly, and the send and send-confirm screens' balances, reserve and
insufficient-balance messages go through truncateAmountNeverZero(). Both the
send-confirm and approval screens render the fee through formatFee(), which
prices the exact fee in USD, so the same fee reads the same on both. Token
balances keep their six-decimal value: it also leaves dust off the balance
list, and keeps the string within the 18 decimals the balance check reads.

Model: opus-5-5

tests/unknownScaleSend.test.js

@@ -389,7 +389,7 @@ describe("a scale the explorer's own rows disagree about", () => {
         expect(txInfo.tokenBalance).toBe("5.0");
         confirmTx.show(txInfo);
         await settle();
-        expect(text("confirm-balance")).toBe("5.0 NOVEL");
+        expect(text("confirm-balance")).toBe("5.0000 NOVEL");
         expect(errors()).toBe("");
         expect(sendDisabled()).toBe(false);
     });
@@ -431,7 +431,7 @@ describe("the confirmation screen tells an unknown balance from a zero one", ()
         const zero = await render("0.0");
         expect(unknown.balance).not.toBe(zero.balance);
         expect(unknown.balance).toBe("unknown (NOVEL)");
-        expect(zero.balance).toBe("0.0 NOVEL");
+        expect(zero.balance).toBe("0.0000 NOVEL");
     });
 
     // Both hit INSUFFICIENT_TOKEN — an unknown balance is treated as nothing to
@@ -443,7 +443,7 @@ describe("the confirmation screen tells an unknown balance from a zero one", ()
         expect(unknown.errors).not.toBe(zero.errors);
         expect(unknown.errors).toContain("This token's balance is unknown");
         expect(unknown.errors).not.toContain("You have");
-        expect(zero.errors).toContain("You have 0.0 NOVEL");
+        expect(zero.errors).toContain("You have 0.0000 NOVEL");
         expect(zero.errors).not.toContain("balance is unknown");
     });
 });