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/colors.py

@@ -0,0 +1,16 @@
+from __future__ import annotations
+
+from .models.note import NOTE_COLORS
+
+# Notes and labels share one colour palette (their sets were identical). NOTE_COLORS
+# is the canonical vocabulary (defined on the model); this module is the single home
+# for the "clamp to the palette" normalizer so notes.py, labels.py and sync.py stop
+# each carrying their own copy.
+
+__all__ = ["NOTE_COLORS", "normalize_color"]
+
+
+def normalize_color(color: object) -> str:
+    """Return `color` if it's a known palette key, else the default. One definition
+    for both notes and labels."""
+    return color if color in NOTE_COLORS else "default"