harden: show a personal message's hex and mark control characters in its text (closes #403)

The signature screen showed only the text a personal message decodes
to, with bidirectional and zero-width characters acting on it, so a
site could make the message read differently from the bytes that are
signed, and a message that was not hex was decoded into NUL
characters. The screen now shows the hex as "Raw data" alongside the
decoded text, and each control or format character in the text as a
U+XXXX mark. Signing takes the bytes from the hex, so a message that
is not hex cannot be signed: it is shown as plain text, with the error
line saying so and "Sign" disabled.

Model: opus-5-5

TODO.md

@@ -45,6 +45,16 @@ but the review is broader than any of them.
 
 # Completed Steps
 
+- 2026-10-04: The signature screen shows a personal message as the bytes that
+  are signed ([#403](https://git.eeqj.de/sneak/AutistMask/issues/403)). It
+  showed only the decoded text, with bidirectional and zero-width characters
+  acting on it, so a site could make the message read differently from what is
+  signed, and a message that was not hex was shown as NUL characters. The hex is
+  now shown as "Raw data" alongside the decoded text, control and format
+  characters in the text are shown as `U+XXXX` marks, and a message that is not
+  hex is shown as plain text with "Sign" disabled, since signing takes the bytes
+  from the hex and such a message has none to sign.
+
 - 2026-10-04: A nonce the site supplies with `eth_sendTransaction` is ignored
   ([#404](https://git.eeqj.de/sneak/AutistMask/issues/404)). It was passed on to
   the transaction, so a site could replace one of the user's pending