fix: the Chrome layout tests and popup comments use the popup's real 396x600 size (closes #556)

POPUP_VIEWPORT in tests/e2e/run.js said 360x600, while body in
src/popup/styles/main.css is 396px wide and the background opens an
approval window at 396x600. It is now 396x600, defined once near the top
of the file, and the hostile token symbol's frame takes its size from it.

assertAddressesFit() now measures the page at that size, then puts back
the size it found. Every page it measured, the popup's own screens and the
two approval prompts, was 1280x720, where a page never scrolls sideways,
so its sideways-scroll check could not fail. README's list of what the
Chrome suite leaves out says what sizing the page does not cover.

Comments in src/popup/index.html and tests/dustThreshold.test.js that
said 360x600 now say 396x600.

Model: opus-5-5

src/popup/index.html

@@ -90,7 +90,7 @@
                 <div id="add-wallet-section-mnemonic">
                     <!-- The die button shares the instruction's row: on a row
                          of its own it pushes Import below the fold at
-                         360x600. -->
+                         396x600. -->
                     <div class="mb-2 flex justify-between items-start gap-2">
                         <p>
                             Enter your 12 or 24 word recovery phrase below, or
@@ -212,7 +212,7 @@
                     />
                 </div>
                 <!-- The error line sits beside Import, not above it: at
-                     360x600 the button already starts near the bottom of
+                     396x600 the button already starts near the bottom of
                      the popup, and a line of its own would push it below
                      the fold. The longest error fits on one line here. -->
                 <div class="flex items-center gap-2">