🚀 Feature: insert standard emojis (😊 ❤️ 👍, etc.)

### Description

The app's circled/squared font variants (🅰 ⒶⒷⒸ 🄰) look emoji-ish but aren't actual emojis — they're styled letters. There's currently no way to insert real standard emojis like 😊 (smile), ❤️ (love), 👍, 🔥 into the text. Users wanting those have to leave the app and use their OS emoji picker, which breaks flow.

Proposal: add an emoji button to the toolbar that opens a small panel of standard emojis (with search + categories like "smileys", "hearts", "hands"). Clicking an emoji inserts it at the cursor position via the existing CodeMirror instance. Font-mapping behavior is unchanged — emojis pass through the font buttons untouched since they aren't in any font table.