Fix session manager formatting issue with Unicode emojis

- Added calculate_display_width() method to properly handle emoji widths
- Fixed overlapping text in session manager UI by accounting for double-width emojis
- Handles compound emojis with variation selectors (like 🗂️)
- Uses saturating_sub() to prevent underflow in padding calculations

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

test_export.json

@@ -1,5 +1,5 @@
 {
-  "exported_at": "2025-08-25T04:01:15.432999997+00:00",
+  "exported_at": "2025-08-31T03:02:51.004770151+00:00",
   "messages": [
     {
       "content": "You are an AI assistant running in a terminal (CLI) environment. Optimise all answers for 80‑column readability, prefer plain text, ASCII art or concise bullet lists over heavy markup, and wrap code snippets in fenced blocks when helpful. Do not emit trailing spaces or control characters.",