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/shared/walletDefects.js

@@ -41,12 +41,10 @@ const DEFECTS = {
                 "changed or removed, and this wallet stays until you delete " +
                 "it yourself.",
         ],
-        // One sentence for the places that have room for one: the flash on a
-        // blocked Send, the inline error on the approval screens.
-        shortMessage:
-            "This wallet cannot sign, because it was imported from an " +
-            "extended private key that is not a master key. The wallet list " +
-            "explains what happened.",
+        // One line, for the flash on a blocked Send and the inline error on
+        // the approval screens. It must fit on the flash line; see showFlash()
+        // in src/popup/views/helpers.js.
+        shortMessage: "This wallet cannot sign. See the wallet list.",
     },
 };