Remove the title field — a note is named by its first line

Operator (note 2897): "notes shouldn't have a title field." The concept of a NAME
stays — search results, export filenames and the command palette all need one —
but nothing is typed into it any more. `display_title` is now the first non-empty
line of the body, falling back to the first checklist item.

That fallback is what step 2 bought, and the reason this could not go first: a
checklist had no body to be named from, so the title was its only name. Now every
note has a body, and a note that is only a checklist is named by its first item.

Gone everywhere: the column and note_revisions.title (0026), the field on the
core's Note/NoteCreateInput/NoteRevision and its SQLite columns (user_version 7),
`normalize_title`, the wire field, the FFI record and `NoteEdit::Title` /
`ClearTitle`, the web editor's "Title (optional)" input and the card's <h3>, and
the Android title field in both the compose sheet and the editor.

**The search vector had to be rebuilt, not just left alone.** `notes.search_vector`
is a STORED GENERATED column whose expression names `title` — Postgres refuses to
drop a column another generated column depends on. It is dropped and recreated over
`display_title` at weight A, which keeps the original intent: a note's NAME ranks
above the rest of its body.

**An imported title becomes the note's first body line.** Keep notes carry one, and
so does any ThoughtSync export taken before this. Dropping it would silently lose
text someone wrote; folding it in puts it exactly where a name now lives, so the
note arrives named as it was. Skipped when the body already opens with that line,
so re-importing an export this code produced doesn't stack duplicates.

Two smaller things fell out. The Android editor loses its bold first field — one
weight throughout, because the first line is the note's name but not a different
KIND of text, which is most of step 4 arriving early. And `ClearTitle`'s
justification comment moved to `ClearRemindAt`, which is now the surviving example
of why NoteEdit is a list rather than a struct of options.

Protocol note corrected to say what actually shipped: v2 is "no kind, no title",
one bump for the pair.

Verified with the local Rust gate this time, not by CI: fmt, clippy and 116 tests
all green before pushing. It caught four things — orphaned serde attributes where
fields were removed, a `wire::Preview.title` I deleted by mistake (a link preview
still has one), nine retention fixtures inserting a dropped column, and four
rustfmt diffs.

src/thoughtsync/models/note.py

@@ -38,11 +38,11 @@ class Note(Base):
     owner_id: Mapped[uuid.UUID] = mapped_column(
         UUID(as_uuid=True), ForeignKey("users.id", ondelete="CASCADE"), nullable=False
     )
-    title: Mapped[str | None] = mapped_column(Text(), nullable=True)
-    # The note's display NAME: explicit title if set, else the first non-empty body
-    # line (see notes.derive_display_title). Persisted so every note — even a body-only
-    # one — has something to be called in search results and in an export filename,
-    # without forcing the user to type a title.
+    # The note's NAME: its first non-empty body line, else its first checklist item
+    # (see notes.derive_display_title). There is no title field to prefer — a note is
+    # a body plus optional items, and this is simply the first thing written in it.
+    # Persisted so search results and export filenames have something to say, and so
+    # the full-text vector can weight it above the rest of the body.
     display_title: Mapped[str] = mapped_column(Text(), nullable=False, server_default="")
     body: Mapped[str] = mapped_column(Text(), nullable=False, server_default="")
     color: Mapped[str] = mapped_column(Text(), nullable=False, server_default="default")
@@ -73,7 +73,6 @@ class Note(Base):
     def serialize(self) -> dict:
         return {
             "id": str(self.id),
-            "title": self.title,
             "display_title": self.display_title,
             "body": self.body,
             "color": self.color,