S1: iso() datetime helper + one shared colour normalizer (colors.py)

M9 section S1, commit 3 — two more shared-toolkit pieces:

- common.iso(dt): the "x.isoformat() if x else None" idiom (repeated 20+ times
  across every serializer) as one helper. Adopted in Note.serialize() and the
  revision serializer; other serializers adopt it in their sections.
- colors.py: NOTE_COLORS (canonical, on the model) + a single normalize_color().
  notes.py now imports the palette + normalizer from here and drops its local
  copy. labels.py's identical LABEL_COLORS/_normalize_label_color fold into this
  in the Organize section (S3); sync in S4.

normalize_color and NOTE_COLORS remain importable from thoughtsync.notes (used by
tests + sync), so nothing downstream breaks. common has no in-app imports, so the
model→common→colors chain has no cycle. Behavior-preserving.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FRgehjoz7Yv8LkUfADxACm

src/thoughtsync/common.py

@@ -7,6 +7,12 @@ from datetime import datetime
 # definition instead of a near-identical copy per module.
 
 
+def iso(dt: datetime | None) -> str | None:
+    """A tz-aware datetime as an ISO-8601 string, or None. Collapses the
+    `x.isoformat() if x else None` idiom repeated across every serializer."""
+    return dt.isoformat() if dt else None
+
+
 def parse_dt(raw: object) -> datetime | None:
     """Parse an ISO-8601 timestamp (accepting a trailing 'Z' for UTC).