fix(email): guard _decode_header against unknown MIME charset (#1354)

A header that declares an unknown or invalid MIME charset (e.g. a malformed
or spam Subject like =?x-unknown-charset?B?...?=) raised an uncaught
LookupError. bytes.decode(..., errors="replace") only handles byte-decode
errors, not codec *lookup* failures, so the "replace" safety net did not
apply.

_decode_header decodes Subject/From/To/Cc for the inbox list, single-message
fetch, and the background mail pollers (routes/email_routes.py,
routes/email_pollers.py, src/builtin_actions.py), so a single bad message
could crash the whole inbox render or the poller loop.

Wrap the per-part decode in try/except (LookupError, ValueError) and fall
back to utf-8/replace. Valid charsets (utf-8, iso-8859-1, ...) are unchanged.

Adds tests/test_email_decode_header.py — the unknown-charset case fails
before this change and passes after.

routes/email_helpers.py

@@ -751,7 +751,13 @@ def _decode_header(raw):
     decoded = []
     for data, charset in parts:
         if isinstance(data, bytes):
-            decoded.append(data.decode(charset or "utf-8", errors="replace"))
+            try:
+                decoded.append(data.decode(charset or "utf-8", errors="replace"))
+            except (LookupError, ValueError):
+                # Unknown/invalid MIME charset (e.g. a malformed or spam header
+                # like =?x-unknown-charset?B?...?=). errors="replace" only covers
+                # byte-decode errors, not codec lookup, so fall back to utf-8.
+                decoded.append(data.decode("utf-8", errors="replace"))
         else:
             decoded.append(data)
     return " ".join(decoded)