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

tests/dustThreshold.test.js

@@ -99,12 +99,13 @@ describe("the flash line the message is shown in", () => {
     // length, including one that wrapped to two lines and pushed the
     // settings view down 12px.
     //
-    // The assertion that actually measures — empty line vs. the message,
-    // real Chromium, documented 360x600 popup — is
-    // "a rejected dust threshold shifts no layout (#233)" in
-    // tests/e2e/run.js, run by make test-e2e. It is not in make check
-    // because REPO_POLICIES.md caps make test at 20 seconds and a browser
-    // suite does not fit; run it before changing the wording.
+    // The line cuts a message too long for it with an ellipsis (see
+    // showFlash() in src/popup/views/helpers.js). The assertions that
+    // measure that, in a real browser at the documented 360x600 popup, are
+    // "a rejected dust threshold shifts no layout (#233)" and "an over-long
+    // flash message keeps to one line (#252)" in tests/e2e/run.js, run by
+    // make test-e2e. They are not in make check because REPO_POLICIES.md
+    // caps make test at 20 seconds and a browser suite does not fit.
     test("reserves its height in the markup", () => {
         const flashLine = POPUP_HTML.match(
             /<div\s+id="flash-msg"\s+class="([^"]*)"/,