## Context
Adding an existing community by relay URL could fail with `Community
rejected: Load failed` even when its WebSocket endpoint was reachable.
The Add Community flow fetched `/api/join-policy` from the WebView, so a
relay without a matching CORS allowance blocked the policy request
before the app could join it.
## Summary
This bug fix fetches join policies through Tauri's native networking
layer for direct URL joins. Invite-code discovery, policy acceptance,
and signed invite claims remain on the WebView path so those operations
can migrate together later.
## Changes
- Uses native networking for Add Community and first-community direct
URL join-policy requests.
- Validates relay schemes, rejects URLs containing credentials, and
refuses redirects.
- Bounds declared and chunked native responses before JSON parsing.
- Preserves existing `404`, non-success status, malformed JSON, and
absent-policy behavior.
- Requires every join-policy caller to choose its transport explicitly.
Public relays using Buzz's default permissive CORS configuration are not
known to be affected.
### Related issue
Related to #2872.
### Testing
#### Reviewer-reproducible examples
End-to-end red/green requires a relay with restrictive CORS and a Buzz
identity authorized to join it.
##### Red: `main`
From a clean checkout of `main`:
```bash
. ./bin/activate-hermit
just staging
```
In Buzz Desktop:
1. Add another community so the restrictive-CORS relay can be removed.
2. Remove that relay.
3. Open Add Community and enter the relay's WebSocket URL.
4. Select Add Community.
Observed result:
```text
Community rejected: Load failed
```
##### Green: this PR
From a clean checkout of this branch:
```bash
. ./bin/activate-hermit
just staging
```
Repeat the same steps above.
Observed result:
```text
The community rejoins successfully.
```
Supporting checks:
- Six native join-policy tests, including oversized declared and chunked
responses.
- Four TypeScript API tests, including the native command contract.
- E2E build and four focused onboarding and sidebar Playwright tests.
- Full `just ci` and pre-push suites.
- Builderbot, Kalvin, and minimize-diff review fanout found no
actionable issues after the final rebase.
