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/deleteAddress.test.js

@@ -4,8 +4,8 @@
 // the copy is the substance and is tested as such. Two things it must not
 // get wrong: what it takes to get the address back — the app refuses both
 // obvious routes — and what counts as holding something, which is any
-// ERC-20 as well as ETH, at any size, including a balance that rounds to
-// zero at the four decimals the balance lines render. The DOM behaviour
+// ERC-20 as well as ETH, at any size, including a balance below the four
+// decimals the balance lines render. The DOM behaviour
 // around them is driven against the real popup by tests/e2e/run.js.
 
 // helpers.js pulls in state.js, which reads chrome.storage.local at load.
@@ -102,7 +102,7 @@ describe("whether an address holds anything", () => {
         expect(addressHoldsFunds(TOKEN_ONLY)).toBe(true);
     });
 
-    // 0.00001 ETH renders as "0.0000" at four decimals. It is still money.
+    // 0.00001 ETH is below four decimals. It is still money.
     test("an ETH balance below the displayed precision counts", () => {
         expect(addressHoldsFunds(DUST)).toBe(true);
     });
@@ -141,13 +141,12 @@ describe("the balance warning on the removal confirmation", () => {
         expect(html).toContain("2500.0000");
     });
 
-    // The rendered line says 0.0000 for this address — that is the display
-    // format, shared with Home and AddressDetail — and the warning is shown
-    // all the same, because the balance is not zero.
-    test("an ETH balance that renders as 0.0000 is warned about", () => {
+    // The rendered line shows the balance to its first nonzero digit, as Home
+    // and AddressDetail do (README.md, Amounts).
+    test("an ETH balance below 0.0001 is warned about, and reads as itself", () => {
         const html = balanceWarningHtml(DUST);
         expect(html).toContain("This address holds a balance.");
-        expect(html).toContain("<span>0.0000</span>");
+        expect(html).toContain("<span>0.00001</span>");
     });
 
     test("an ETH balance no refresh has read is warned about, and reads as unknown", () => {