v0.166.0: mobile nav off-canvas drawer + sidebar cleanup + versioned logo/favicon URLs

Mobile nav was broken — the <=768px block predated the v0.146.0 accordion and
flattened .nav-links into a horizontal overflow-x strip, clipping the accordion's
nested sub-lists. Replaced with a sticky top bar + off-canvas left drawer that
reuses the vertical sidebar (accordion untouched), plus a no-js static fallback.
Removed the sidebar customer-name span (kept on login). Added ?v={{.Version}}
cache-bust to logo/favicon URLs (Cloudflare 4h edge-cache; 0.126.1 failure mode).

Part 4 (outlined-logo constant swap) gated out per §3a: live felhom.eu main still
serves a logo.svg with live <text>/font-family; constants unchanged.

5 new tests via the real layout/CSS render; nav_accordion invariants unchanged.

controller/internal/web/auth.go

@@ -327,6 +327,7 @@ func (s *Server) renderLogin(w http.ResponseWriter, errorMsg, flashMsg string) {
 	data := map[string]interface{}{
 		"Title":        "Bejelentkezés",
 		"CustomerName": s.cfg.Customer.Name,
+		"Version":      s.version, // logo ?v= cache-bust (v0.166.0)
 		"Error":        errorMsg,
 		"Flash":        flashMsg,
 	}