Add HQ and station icons; redesign tunnel build-button icon

Add hq.svg (flag-base command building) and station.svg (shield, rendered
glyph-only over each side's fill) for in-world building icons. Redraw
tunnel_entry.svg as the shared Tunnel build button: the belt runs on the
surface at both ends and drops straight into/out of the ground, with rounded
corners and a single exit tip. No belt/splitter/tunnel world icons.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Y7N59FsLA5e2kuVdqe4Uhc

bin/app/data/icons/buildings/tunnel_entry.svg

@@ -1,6 +1,6 @@
 <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 100 100" width="100" height="100">
   <rect width="100" height="100" rx="22" fill="#4f7562"/>
   <g transform="translate(13,13) scale(2.3125)" fill="none" stroke="#ffffff" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">
-    <path d="M4 9h24"/><path d="M8 9C8 24 24 24 24 9"/><path d="M14.5 18.5 17.5 20 14.5 21.5"/>
+    <path d="M3 13H29"/><path d="M4 7H8Q10 7 10 9.5V21.5Q10 24 12.5 24H19.5Q22 24 22 21.5V9.5Q22 7 24 7H27"/><path d="M24.5 4.5 27 7 24.5 9.5"/>
   </g>
 </svg>