fix(welcome): gate Gemini button on availability, drop unconditional tunnel button

- Remove the always-visible Cloudflare Tunnel welcome button and QR
  widget; offering it regardless of whether cloudflared is installed
  is a bad default.
- Hide the "Run Gemini" welcome button by default and only show it
  when /api/gemini/status reports available:true, via new
  loadGeminiAvailability() called from showWelcome().

src/web/public/terminal-ui.js

@@ -1195,6 +1195,7 @@ Object.assign(CodemanApp.prototype, {
     if (overlay) {
       overlay.classList.add('visible');
       this.loadTunnelStatus();
+      this.loadGeminiAvailability();
       this.loadHistorySessions();
       this.initSearchPanel();
     }