remove experimental web UI

The web UI shipped as "experimental" and was never finished — no nav
off the dashboard, no live updates, no settled design, never a
supported surface. It was opt-in by default already; leaving the code
in the tree for v0.1.0 only invited "does this work?" questions and
kept HostSummary/BangerSummary/SudoStatus types on the public RPC
surface that nothing else uses.

Removed:

  internal/webui/                         (all Go + templates + assets)
  internal/daemon/web.go                  (server start / Layout / Config / ListVMs / ListImages)
  internal/daemon/dashboard.go            (DashboardSummary aggregator)

Simplified:

  internal/api/types.go                   drop WebURL on PingResult, drop
                                          HostSummary / SudoStatus / BangerSummary /
                                          DashboardSummary / DashboardSummaryResult
  internal/model/types.go                 drop DaemonConfig.WebListenAddr
  internal/config/config.go               drop web_listen_addr from fileConfig + Load
  internal/daemon/daemon.go               drop webListener / webServer / webURL fields +
                                          startWebServer() call + ping WebURL population
  internal/cli/banger.go                  `daemon status` output no longer branches on web
  internal/daemon/{doc.go,ARCHITECTURE.md} drop web UI sections
  README.md                               drop web_listen_addr config bullet + security paragraph

Tests updated to reflect the new shape. Coverage 57.3 -> 58.9% (the
webui package was largely untested; its removal lifts the ratio
without moving the numerator). `banger daemon status` output and
--help are web-free. Lint + full suite green.

internal/model/types.go

@@ -53,7 +53,6 @@ const (
 
 type DaemonConfig struct {
 	LogLevel            string
-	WebListenAddr       string
 	FirecrackerBin      string
 	SSHKeyPath          string
 	AutoStopStaleAfter  time.Duration