fix: Bug - Emojis should be user bound not client bound

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>

toju-app/src/app/infrastructure/persistence/README.md

@@ -45,7 +45,7 @@ Both backends store the same entity types:
 | `users` | `oderId` | | User profiles |
 | `rooms` | `id` | | Server/room metadata |
 | `reactions` | `oderId-emoji-messageId` | | Emoji reactions, deduplicated per user |
-| `customEmojis` / `custom_emojis` | `id` | `updatedAt`, `creatorUserId` | Known custom emoji image assets synced over peer data channels; `savedByUser` controls picker/library membership |
+| `customEmojis` / `custom_emojis` | `id` | `updatedAt`, `creatorUserId` | Known custom emoji image assets synced over peer data channels. Asset store only (the Electron table is shared across local accounts); picker/library membership is **user-bound**, tracked per user id in `localStorage` (`metoyou_custom_emoji_saved:<userId>`), not by the row's legacy `savedByUser` flag |
 | `bans` | `oderId` | | Active bans per room |
 | `attachments` | `id` | | File/image metadata tied to messages |
 | `meta` | `key` | | Key-value pairs (e.g. `currentUserId`) |