fix: keep every flash message on the one line it reserves (closes #252)

The flash line reserves one line of height, so a message that wrapped
pushed the screen below it down. Every message is now at most 50
characters, one line of the popup's monospace font: the longer ones are
reworded, and the ones that carried a wallet name or text from a server
no longer do. The rule is written at showFlash().

A new end-to-end test drives the longest message and fails if the
line's rendered height grows. It measures in the monospace font, which
Firefox draws and Chromium does not.

Model: opus-5-5

tests/e2e/run.js

@@ -1400,6 +1400,63 @@ test("a rejected dust threshold shifts no layout (#233)", async (env) => {
     }
 });
 
+// ------------------------------------------------ the flash line (#252)
+
+// Every flash message must fit the one line #flash-msg reserves (see
+// showFlash() in src/popup/views/helpers.js). This drives the longest one
+// and measures the line's height with it.
+//
+// The line is measured in the monospace font the popup declares. Firefox
+// draws the popup in it; Chromium draws it in the system font instead
+// (https://git.eeqj.de/sneak/AutistMask/issues/418), which is narrower, so a
+// message that wraps in Firefox would still fit here and the test would pass.
+test("the longest flash message fits on one line (#252)", async (env) => {
+    const page = await openPopup(env.ctx, env.popupUrl);
+    try {
+        await page.setViewportSize(POPUP_VIEWPORT);
+        await openSettings(page);
+        await page.click("#btn-settings-add-token");
+        await visible(page, "#view-settings-addtoken");
+
+        const font = await page.evaluate(() => {
+            const line = document.getElementById("flash-msg");
+            line.style.fontFamily = "var(--font-mono)";
+            return getComputedStyle(line).fontFamily;
+        });
+        assert(
+            font.includes("monospace"),
+            "the flash line is not in the monospace font: " + font,
+        );
+
+        const before = await page.evaluate(measureFlashLine);
+        await page.fill("#settings-addtoken-address", "not an address");
+        await page.click("#btn-settings-addtoken-manual");
+        const after = await waitForFilledFlashLine(page);
+
+        assert(
+            after.flashHeight === before.flashHeight,
+            "the flash line is " +
+                before.flashHeight +
+                "px empty and " +
+                after.flashHeight +
+                "px with " +
+                JSON.stringify(after.text) +
+                ", so the message wraps",
+        );
+        assert(
+            after.text === "Enter a valid contract address starting with 0x.",
+            "the screen flashed " +
+                JSON.stringify(after.text) +
+                ", not the message this test measures",
+        );
+
+        await page.click("#btn-settings-addtoken-back");
+        await visible(page, "#view-settings");
+    } finally {
+        await page.close();
+    }
+});
+
 // --------------------------------------------- confirmation screen (#238)
 //
 // The screen that decides what gets signed. The arithmetic underneath it