WEB UI

[GIC-de]

Summary

Adds an experimental, embedded single-page web UI (pkg/server/webui/) for
creating and observing BNG Blaster test instances without calling the REST
API directly, plus the supporting API surface it needs:

• New read endpoints: /_overview, /_streams, /_sessions, /_logs,
  /_files (+ per-file download), and /api/v1/interfaces (host network
  interfaces, used to populate the "New Instance" form)
• Embedded OpenAPI/Swagger docs served under /docs
• Response caching for the summary endpoints (pkg/server/cache.go) to
  avoid hammering the bngblaster control socket on repeated UI polling

The web UI, the interfaces endpoint, and the upload endpoint are new attack
surface with no authentication yet, so all three ship disabled by
default behind -ui, -interfaces-api, and -upload flags (see the new
"Experimental Web UI" section in the README for how to turn them on).

Also includes some incidental fixes picked up along the way: corrected the
default bngblaster executable path, and config/logging tweaks for the
systemd packaging.

Test plan

• go build ./...
• go test ./...
• Manual smoke test of the UI in a browser (create/start/stop an
  instance, check overview/streams/sessions/logs/files views, upload a
  file) — recommended before merging since this is a large new surface
  with no automated UI tests

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