app: replace regex html stripping with stdlib parser

Handles malformed tags, nested markup, and CDATA that the
naive regex missed.

src/tuimble/app.py

@@ -5,6 +5,7 @@ from __future__ import annotations
 import html
 import logging
 import time
+from html.parser import HTMLParser
 
 from textual import events, on, work
 from textual.app import App, ComposeResult
@@ -991,9 +992,22 @@ class TuimbleApp(App):
         self.exit()
 
 
+class _HTMLStripper(HTMLParser):
+    """Extract text content from HTML, discarding all tags."""
+
+    def __init__(self):
+        super().__init__()
+        self._parts: list[str] = []
+
+    def handle_data(self, data: str):
+        self._parts.append(data)
+
+    def get_text(self) -> str:
+        return "".join(self._parts)
+
+
 def _strip_html(text: str) -> str:
     """Remove HTML tags and unescape entities from Mumble messages."""
-    import re
-
-    clean = re.sub(r"<[^>]+>", "", text)
-    return html.unescape(clean)
+    stripper = _HTMLStripper()
+    stripper.feed(text)
+    return html.unescape(stripper.get_text())