fix: v0.10.2 drag-select stays highlighted and copies

A left drag in the pane is turned into xterm's own selection (as Shift+drag
does) instead of tmux copy-mode, which dropped the highlight on release.
Copy on release with a toast; Ctrl+C copies when something is selected.
Wheel scroll and status-line clicks still reach tmux. Closes #51

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WHRLCCdXYm41Qx5kNhehaP

web/index.template.html

@@ -176,7 +176,7 @@
     <span class="lt-footer-sep">│</span>
     <span class="lt-footer-hint"><span class="lt-footer-key">^b d</span>detach</span>
     <span class="lt-footer-sep">│</span>
-    <span class="lt-footer-hint"><span class="lt-footer-key">⇧drag</span>select</span>
+    <span class="lt-footer-hint"><span class="lt-footer-key">drag</span>copy</span>
     <span class="lt-footer-sep">│</span>
     <span class="lt-footer-hint"><span class="lt-footer-key">drop / paste</span>attach files</span>
     <span class="lt-footer-sep">│</span>