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, and the fee no longer
matched the approval screen's. The ETH balance is now stored exactly, and the
send screen's Current balance and the send-confirm screen's balance, fee,
reserve and insufficient-balance messages go through truncateAmountNeverZero(),
as the approval screen does. Token balances keep their six-decimal value: it is
also what 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");
     });
 });