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/daemon/doc.go

@@ -1,9 +1,8 @@
 // Package daemon hosts the Banger daemon process.
 //
-// The daemon exposes a JSON-RPC endpoint over a Unix socket and, optionally,
-// an experimental local web UI. It owns VM lifecycle, image management,
-// guest sessions, host networking bootstrap, and state persistence via
-// internal/store.
+// The daemon exposes a JSON-RPC endpoint over a Unix socket. It owns VM
+// lifecycle, image management, guest sessions, host networking bootstrap,
+// and state persistence via internal/store.
 //
 // The package is organised into cohesive groups. Pure stateless helpers for
 // each group have been lifted into subpackages; orchestrator methods
@@ -68,11 +67,9 @@
 // Core (in this package):
 //
 //	daemon.go         Daemon struct, Open/Close/Serve, dispatch
-//	dashboard.go      dashboard metrics aggregation
 //	doctor.go         host diagnostics
 //	logger.go         slog configuration
 //	runtime_assets.go paths to bundled companion binaries
-//	web.go            experimental local web UI server
 //
 // Lock ordering:
 //