m4.5: titles optional — every note gets an auto display name

Capture starts in the body, so forcing a title feels odd and body-only
notes had no name — which made them unlinkable. Fix both: persist a
display_title = explicit title if set, else the note's first non-empty
body line (deterministic, no AI). The title field stays optional.

- migration 0012: notes.display_title (NOT NULL, best-effort backfill;
  the app recomputes precisely on next save)
- derive_display_title() helper, set on create + update
- drive the /titles index, backlinks, graph edges + node labels, and
  [[wiki-link]] resolution off display_title so body-only notes are
  nameable, findable (command palette / [[ autocomplete), and linkable
- rename-repoint generalized: inbound [[Old Name]] links now survive a
  name change via the first body line too, not just an explicit title
- unit tests for the derivation (explicit wins, first non-empty line,
  blank/empty, length cap)
- frontend: display_title on the Note type; title field placeholder now
  reads "Title (optional)"

First item of M4.5 (frictionless input & recall); unblocks the linking
work. Card rendering unchanged (no first-line duplication).

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

frontend/src/components/NoteEditor.vue

@@ -279,7 +279,7 @@ async function act(fn: () => Promise<void>) {
         <input
           v-model="title"
           type="text"
-          placeholder="Title"
+          placeholder="Title (optional)"
           class="w-full bg-transparent text-base font-semibold outline-none placeholder:text-neutral-400"
         />
         <div v-if="liveNote.kind === 'text'" class="relative">