feat(view): zoom-window ribbon button and bare ZOOM command

Bare `ZOOM` had no dispatch arm — only the sub-keyword forms (ZOOM
EXTENTS/IN/OUT/WINDOW/…) were handled, so typing `ZOOM` fell through to
an unknown-command error. It now starts the interactive window zoom.

Add the missing Zoom Window ribbon button: the command existed but had no
tool()/icon, so it was reachable only by typing. Wire it into View ▸
Navigate with a new dashed-rectangle magnifier icon.

Closes #210

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

assets/icons/zoom_window.svg

@@ -0,0 +1,5 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" width="64" height="64" fill="none" stroke-linecap="round" stroke-linejoin="round">
+  <circle cx="11" cy="11" r="7" stroke="#e0e0e0" stroke-width="1.5"/>
+  <line x1="16.5" y1="16.5" x2="21" y2="21" stroke="#e0e0e0" stroke-width="1.5"/>
+  <rect x="7.5" y="8" width="7" height="6" stroke="#4cc9f0" stroke-width="1.5" stroke-dasharray="2 1.5"/>
+</svg>