fix(backend): use system chromium for agent-browser on all architectures (#12473)

## Summary

- Replaces the arch-conditional chromium install (ARM64 vs AMD64) with a
single approach: always use the distro-packaged `chromium` and set
`AGENT_BROWSER_EXECUTABLE_PATH=/usr/bin/chromium`
- Removes `agent-browser install` entirely (it downloads Chrome for
Testing, which has no ARM64 binary)
- Removes the `entrypoint.sh` wrapper script that was setting the env
var at runtime
- Updates `autogpt_platform/db/docker/docker-compose.yml`: removes
`external: true` from the network declarations so the Supabase stack can
be brought up standalone (needed for the Docker integration tests in the
test plan below — without this, `docker compose up` fails unless the
platform stack is already running); also sets
`GOTRUE_MAILER_AUTOCONFIRM: true` for local dev convenience (no SMTP
setup required on first run — this compose file is not used in
production)
- Updates `autogpt_platform/docker-compose.platform.yml`: mounts the
`workspace` volume so agent-browser results (screenshots, snapshots) are
accessible from other services; without this the copilot workspace write
fails in Docker

## Verification

Tested via Docker build on arm64 (Apple Silicon):
```
=== Testing agent-browser with system chromium ===
✓ Example Domain
  https://example.com/
=== SUCCESS: agent-browser launched with system chromium ===
```
agent-browser navigated to example.com in ~1.5s using system chromium
(v146 from Debian trixie).

## Test plan

- [x] Docker build test on arm64: `agent-browser open
https://example.com` succeeds with system chromium
- [x] Verify amd64 Docker build still works (CI)