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/balanceLineEscaping.test.js

@@ -41,13 +41,13 @@ describe("balanceLine", () => {
         // this payload before its id attribute, so an assertion about the
         // rest of the payload would pass on the cap and say nothing about
         // the escape.
-        const html = balanceLine(HOSTILE_SYMBOL, 1, null, null);
+        const html = balanceLine(HOSTILE_SYMBOL, "1", null, null);
         expect(html).not.toContain("<iframe");
         expect(html).toContain("&lt;iframe");
     });
 
     test("caps the symbol before rendering it", () => {
-        const html = balanceLine("A".repeat(4096), 1, null, null);
+        const html = balanceLine("A".repeat(4096), "1", null, null);
         expect(html).toContain("A".repeat(MAX_SYMBOL_LENGTH - 1) + "…");
         expect(html).not.toContain("A".repeat(MAX_SYMBOL_LENGTH + 1));
     });
@@ -55,13 +55,13 @@ describe("balanceLine", () => {
     // The token id lands inside data-token="...", so a quote in it is a
     // way out of the attribute and into a new one.
     test("keeps a quote-bearing token id inside its attribute", () => {
-        const html = balanceLine("TKN", 1, null, '" onclick="alert(1)');
+        const html = balanceLine("TKN", "1", null, '" onclick="alert(1)');
         expect(html).not.toContain('onclick="');
         expect(html).toContain('data-token="&quot; onclick=&quot;alert(1)"');
     });
 
     test("renders an ordinary holding unchanged", () => {
-        const html = balanceLine("USDC", 1.5, null, "0xabc");
+        const html = balanceLine("USDC", "1.5", null, "0xabc");
         expect(html).toContain("<span>USDC</span>");
         expect(html).toContain("<span>1.5000</span>");
         expect(html).toContain('data-token="0xabc"');
@@ -69,7 +69,7 @@ describe("balanceLine", () => {
 
     // formatUsd() writes a value under a cent as "< $0.01".
     test("escapes the USD value along with the symbol", () => {
-        const html = balanceLine("USDC", 0.001, 1, null);
+        const html = balanceLine("USDC", "0.001", 1, null);
         expect(html).toContain("&lt; $0.01");
         expect(html).not.toContain("< $0.01");
     });