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.

alembic/versions/0026_drop_note_title.py

@@ -0,0 +1,82 @@
+"""drop notes.title and note_revisions.title — a note's name is its first line
+
+Revision ID: 0026
+Revises: 0025
+Create Date: 2026-08-22
+
+M13 step 3. A note is a body plus optional checkable items; its NAME is the first
+non-empty line of that body, falling back to its first checklist item. There is no
+separate field to type into, and `display_title` (already persisted, already what
+search results and export filenames read) carries the name.
+
+## The search vector has to be rebuilt, not just left alone
+
+`notes.search_vector` is a STORED GENERATED column whose expression names `title`
+(migration 0005, weight A) — Postgres will refuse to drop a column another generated
+column depends on, and even if it didn't, the weighting would be wrong. So it is
+dropped and recreated over `display_title` instead, which keeps the original
+intent: the note's NAME ranks above the rest of its body.
+
+Rebuilding a stored generated column re-computes every row, and the GIN index is
+rebuilt with it. On a personal instance that is milliseconds; it is worth knowing
+before running this against something large.
+
+## What happens to existing titles
+
+Nothing preserves them, deliberately: `display_title` was already derived from the
+title when one was set, so every note keeps the NAME it had. What is lost is the
+distinction between "this note has an explicit title" and "this note's first line is
+its name" — which is the distinction being removed.
+
+Imports are the exception and are handled in code, not here: a Keep note's title, or
+one in an export taken before this, is folded in as the note's first body line rather
+than dropped (see `_create_imported_note`).
+"""
+from alembic import op
+import sqlalchemy as sa
+
+revision = "0026"
+down_revision = "0025"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    # Order matters: the generated column depends on `title`, so it goes first.
+    op.execute("DROP INDEX IF EXISTS ix_notes_search")
+    op.execute("ALTER TABLE notes DROP COLUMN IF EXISTS search_vector")
+
+    op.drop_column("notes", "title")
+    op.drop_column("note_revisions", "title")
+
+    op.execute(
+        """
+        ALTER TABLE notes ADD COLUMN search_vector tsvector
+        GENERATED ALWAYS AS (
+            setweight(to_tsvector('english', coalesce(display_title, '')), 'A') ||
+            setweight(to_tsvector('english', coalesce(body, '')), 'B')
+        ) STORED
+        """
+    )
+    op.execute("CREATE INDEX ix_notes_search ON notes USING GIN (search_vector)")
+
+
+def downgrade() -> None:
+    op.execute("DROP INDEX IF EXISTS ix_notes_search")
+    op.execute("ALTER TABLE notes DROP COLUMN IF EXISTS search_vector")
+
+    # Comes back empty. The text is not gone — it is the first line of every body —
+    # but which notes once had an explicit title is not recorded anywhere.
+    op.add_column("notes", sa.Column("title", sa.Text(), nullable=True))
+    op.add_column("note_revisions", sa.Column("title", sa.Text(), nullable=True))
+
+    op.execute(
+        """
+        ALTER TABLE notes ADD COLUMN search_vector tsvector
+        GENERATED ALWAYS AS (
+            setweight(to_tsvector('english', coalesce(title, '')), 'A') ||
+            setweight(to_tsvector('english', coalesce(body, '')), 'B')
+        ) STORED
+        """
+    )
+    op.execute("CREATE INDEX ix_notes_search ON notes USING GIN (search_vector)")