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.

core/src/sync/pull.rs

@@ -240,12 +240,11 @@ fn upsert_note(conn: &Connection, note: &wire::Note) -> rusqlite::Result<()> {
     // `created_at` is deliberately absent from the UPDATE clause: a note's birth time
     // never changes, and the server's copy is the same value anyway.
     conn.execute(
-        "INSERT INTO notes (id, title, body, color, position, pinned, archived,
+        "INSERT INTO notes (id, body, color, position, pinned, archived,
                             trashed, remind_at, recurrence, created_at, updated_at,
                             sync_revision, trashed_at, dirty)
-              VALUES (?1, ?2, ?3, ?4, ?5, ?6, ?7, ?8, ?9, ?10, ?11, ?12, ?13, ?14, 0)
+              VALUES (?1, ?2, ?3, ?4, ?5, ?6, ?7, ?8, ?9, ?10, ?11, ?12, ?13, 0)
          ON CONFLICT(id) DO UPDATE SET
-              title         = excluded.title,
               body          = excluded.body,
               color         = excluded.color,
               position      = excluded.position,
@@ -260,7 +259,6 @@ fn upsert_note(conn: &Connection, note: &wire::Note) -> rusqlite::Result<()> {
               dirty         = 0",
         params![
             note.id,
-            note.title,
             note.body,
             note.color,
             note.position,
@@ -496,7 +494,6 @@ mod tests {
     fn note(id: &str, revision: i64) -> wire::Note {
         wire::Note {
             id: id.to_string(),
-            title: Some("Title".into()),
             body: "Body".into(),
             color: "default".into(),
             position: 0,