labels: per-label color

Give labels a color (migration 0011 adds labels.color, server_default
'default' so existing labels keep the neutral chip). The PATCH endpoint now
updates name and/or color; note serialization carries each label's color.
Frontend: a swatch picker per label in the Edit-labels modal, colored chips
on cards + in the editor (LABEL_CHIP_CLASSES), and a color dot on each
sidebar label. Reuses the note color vocabulary. (Graph node coloring rides
this in the graph-liveliness task.)

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

src/thoughtsync/models/label.py

@@ -19,6 +19,7 @@ class Label(Base):
         UUID(as_uuid=True), ForeignKey("users.id", ondelete="CASCADE"), nullable=False
     )
     name: Mapped[str] = mapped_column(Text(), nullable=False)
+    color: Mapped[str] = mapped_column(Text(), nullable=False, default="default", server_default="default")
     created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), nullable=False, server_default=func.now())