M3 reminders: notes.remind_at + Reminders view

- Migration 0010: notes.remind_at (nullable tz). PATCH accepts remind_at
  (ISO set / null clear); GET /api/notes/reminders (soonest first, non-trashed);
  serialize includes remind_at.
- Frontend: datetime util (local<->ISO, format, overdue); notes store setReminder;
  editor datetime-local picker + clear; card reminder chip (overdue = red);
  sidebar Reminders entry + /reminders view.

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

src/thoughtsync/models/note.py

@@ -48,6 +48,8 @@ class Note(Base):
     archived: Mapped[bool] = mapped_column(Boolean(), nullable=False, server_default=func.false())
     # Soft delete: non-null => in Trash. Restore sets it back to null.
     deleted_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)
+    # Optional reminder time (surfaced in the Reminders view; no push in M3).
+    remind_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)
     created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), nullable=False, server_default=func.now())
     updated_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), nullable=False, server_default=func.now(), onupdate=func.now()
@@ -64,6 +66,7 @@ class Note(Base):
             "pinned": self.pinned,
             "archived": self.archived,
             "trashed": self.deleted_at is not None,
+            "remind_at": self.remind_at.isoformat() if self.remind_at else None,
             "created_at": self.created_at.isoformat() if self.created_at else None,
             "updated_at": self.updated_at.isoformat() if self.updated_at else None,
         }