browse: Playwright Chromium does not honor standard proxy env vars for page navigation

## Summary

`gstack browse` runs inside an environment with standard proxy env vars set (`HTTP_PROXY`, `HTTPS_PROXY`, `ALL_PROXY`, `NO_PROXY`), but page navigation still fails because the Playwright Chromium launch paths do not explicitly pass proxy settings.

## Observed behavior

On a machine where:
- Claude Code itself can use the global proxy successfully
- `curl` through the same proxy succeeds
- local proxy endpoints are healthy

these commands fail with browser-level navigation errors:

```bash
browse goto https://www.google.com
browse goto https://duckduckgo.com
```

Error:

```text
net::ERR_CONNECTION_RESET
```

## Root cause

`browse/src/browser-manager.ts` launches Chromium via Playwright in multiple code paths, but does not pass Playwright's `proxy` option derived from the standard env vars:

- `chromium.launch(...)`
- `chromium.launchPersistentContext(...)`
- handoff / context recreation `launchPersistentContext(...)`

As a result, some surrounding process/network logic may see proxy env vars, but the actual Chromium page-navigation layer does not reliably use them.

## Expected behavior

If standard proxy env vars are present, `gstack browse` should use them for Chromium navigation too.

## Proposed fix

Add a small helper in `BrowserManager` that reads:
- `HTTPS_PROXY` / `https_proxy`
- `HTTP_PROXY` / `http_proxy`
- `ALL_PROXY` / `all_proxy`
- `NO_PROXY` / `no_proxy`

and inject the resulting Playwright `proxy` config into all Chromium launch paths.

## Validation

After patching and rebuilding locally, the following succeeded:

```bash
browse goto https://www.google.com # 200
browse goto https://duckduckgo.com # 200
```