Bind custom emoji library membership to the signed-in user instead of the
client. CustomEmojiService now tracks saved emoji ids per user id in
localStorage (metoyou_custom_emoji_saved:<userId>) and the picker only shows
the active user's set, seeded on first load from legacy savedByUser rows the
user created. This stops a second account on the same client (or Electron's
shared SQLite database) from inheriting another user's emoji picker, while
keeping synced assets available for message rendering.
Adds unit coverage for per-user scoping and a single-page-load Playwright e2e
that switches users client-side (second user joins the first user's server)
and asserts no library leak.
Co-authored-by: Cursor <cursoragent@cursor.com>
