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.

tests/test_email_decode_header.py

@@ -0,0 +1,51 @@
+"""Regression tests for routes.email_helpers._decode_header.
+
+A single email whose Subject/From/To/Cc header declares an unknown or invalid
+MIME charset (e.g. `=?x-unknown-charset?B?...?=`, common in spam/malformed mail)
+used to raise an uncaught LookupError, because `bytes.decode(..., errors="replace")`
+only handles byte-decode errors — not codec *lookup* failures. That crash
+propagated into the inbox list endpoint, message fetch, and the background mail
+pollers (routes/email_routes.py, routes/email_pollers.py, src/builtin_actions.py),
+so one bad message could take down the whole inbox render / poller loop.
+
+These pin the fallback so a bogus charset degrades gracefully to utf-8.
+"""
+import os
+import tempfile
+from pathlib import Path
+
+_tmp_data = Path(tempfile.mkdtemp(prefix="odysseus_decode_hdr_"))
+os.environ.setdefault("DATA_DIR", str(_tmp_data))
+os.environ.setdefault("DATABASE_URL", f"sqlite:///{_tmp_data / 'app.db'}")
+
+from routes.email_helpers import _decode_header
+
+
+def test_unknown_charset_does_not_raise():
+    # The regression: an unknown codec name must not raise LookupError.
+    assert _decode_header("=?x-unknown-charset?B?aGVsbG8=?=") == "hello"
+
+
+def test_invalid_charset_falls_back_to_utf8():
+    # A made-up charset on non-ASCII bytes should still produce a string.
+    raw = "=?totally-bogus?Q?caf=C3=A9?="
+    out = _decode_header(raw)
+    assert isinstance(out, str)
+    assert "caf" in out
+
+
+def test_valid_utf8_unchanged():
+    assert _decode_header("=?utf-8?B?SGVsbG8gV29ybGQ=?=") == "Hello World"
+
+
+def test_valid_iso8859_1_unchanged():
+    assert _decode_header("=?iso-8859-1?Q?caf=E9?=") == "café"
+
+
+def test_plain_ascii_passthrough():
+    assert _decode_header("Just a subject") == "Just a subject"
+
+
+def test_empty_and_none():
+    assert _decode_header("") == ""
+    assert _decode_header(None) == ""