fix: every screen cuts amounts the same way (closes #757)

The balance lists and Home's ETH line rounded with toFixed(4), so 0.99996
ETH read 1.0000 there and 0.9999 on Send, and 0.00004 USDC read 0.0000.
They now show the stored decimal string through truncateAmountNeverZero(),
as Send and the approval screens do. History rows take the same floor.
truncateAmount() is no longer exported. balanceLine() now takes the stored
balance string instead of a parsed number.

Model: opus-5-5

tests/unknownScaleDisplay.test.js

@@ -96,7 +96,7 @@ afterEach(() => {
 describe("balanceLine", () => {
     test("an unknown quantity and a zero one render differently", () => {
         const unknown = balanceLine("NOVEL", null, null, NOVEL);
-        const zero = balanceLine("NOVEL", 0, null, NOVEL);
+        const zero = balanceLine("NOVEL", "0", null, NOVEL);
         expect(unknown).not.toBe(zero);
         expect(quantities(unknown)).toEqual(["NOVEL", "quantity unknown"]);
         expect(quantities(zero)).toEqual(["NOVEL", "0.0000"]);
@@ -105,7 +105,7 @@ describe("balanceLine", () => {
     test("an unknown quantity produces no fiat figure, a zero one does", () => {
         prices[NOVEL] = 3;
         const unknown = balanceLine("NOVEL", null, 3, NOVEL);
-        const zero = balanceLine("NOVEL", 0, 3, NOVEL);
+        const zero = balanceLine("NOVEL", "0", 3, NOVEL);
         // A price times an unknown quantity is not $0.00: that is the same
         // claim of "nothing here" the quantity cell just refused to make.
         expect(unknown).toContain(