fix: keep the flash line to its one line at any message length (closes #252)

The flash line reserves one line, so a message that wrapped pushed the
screen below it down. #flash-msg no longer wraps: text too long for it is
cut with an ellipsis, and showFlash() puts the whole message in its title.
Every message is also reworded to at most 50 characters so none is cut,
and the add-token screens flash a fixed line for any error other than the
two lookup messages, logging the detail.

A new end-to-end test writes a message several lines long into the line
and fails if the line or the screen below it moves.

Model: opus-5-5

src/popup/views/helpers.js

@@ -223,15 +223,19 @@ function clearFlash() {
         flashTimer = null;
     }
     $("flash-msg").textContent = "";
+    $("flash-msg").title = "";
 }
 
+// The flash line reserves exactly one line, and a message that wrapped would
+// push the screen below it down (README, No Layout Shift). So #flash-msg never
+// wraps: text too long for the line is cut with an ellipsis, and the whole
+// message is also put in the line's title. Write messages to fit, at most 50
+// characters, so none is cut.
 function showFlash(msg, duration = 2000) {
     clearFlash();
     $("flash-msg").textContent = msg;
-    flashTimer = setTimeout(() => {
-        $("flash-msg").textContent = "";
-        flashTimer = null;
-    }, duration);
+    $("flash-msg").title = msg;
+    flashTimer = setTimeout(clearFlash, duration);
 }
 
 // A stored token balance as a number, or null when there is no number in it.