[Chore] Remove old embedded UI

## Summary
Remove the legacy **embedded UI** that ships inside the BanyanDB binary / distribution.

## Motivation
Canopy (#13991–#13993, /issues?q=is%3Aissue+canopy) is now the supported web console for BanyanDB. The old embedded UI is redundant, increases the binary and distribution size, and is no longer maintained. Removing it eliminates a class of legacy UI bugs and forces users onto Canopy.

## Scope
- Identify the embedded UI assets and serving code:
- Likely under `banyand/internal/serving` / `pkg/` (HTTP server static handlers).
- Web assets bundled with the binary or under `ui/`, `dist/`, `assets/` if any.
- CLI flags, config keys, and env vars that enable the embedded UI.
- Remove the serving endpoints, assets, build steps, and CLI flags.
- Update docs (`docs/operation/observability/`, `docs/api-reference.md`, README, CHANGES.md) to point at Canopy as the replacement.
- Make sure the binary still builds and runs without the embedded UI (it should — Canopy is a separate process).

## Tasks
1. **Research** — locate the embedded UI code path, its assets, its flags, and any docs that mention it.
2. **Removal** — delete the code, assets, build steps, and flags.
3. **Tests** — keep CI green; update or remove any e2e tests that exercised the embedded UI.
4. **Migration note** — add a CHANGES.md entry and a doc pointer to Canopy.
5. **Docs** — sweep references and links.

## Acceptance Criteria
- No embedded UI code, assets, or flags remain in the BanyanDB binary.
- Binary size reduced.
- Docs consistently recommend Canopy for the web console.
- CHANGES.md notes the removal in the 0.12.0 release notes.

## Related
- Parent milestone: BanyanDB - 0.12.0 (#264)
- Replacement UI: Canopy

Start by tracing the embedded UI serving code and assets under banyand/internal/serving, pkg/, ui/, dist/, or assets/, then locate related CLI flags, configuration, environment variables, build steps, and e2e tests. Review the listed documentation and CHANGES.md; done means the embedded UI code, assets, flags, and references are removed, Canopy is documented as the replacement, and the binary and CI still work.

Written by the indexing model from the issue text.