Add connection button routes to AI providers page instead of MCP server form

### Before filing

- [x] I searched [open and closed issues](https://github.com/block/berd/issues?q=is%3Aissue) for duplicates.
- [x] I reproduced this on the [latest release](https://github.com/block/berd/releases).
- [x] This is one bug, not several bundled together.

### Closest existing issue

none found

### What's broken

Settings → Connections → Local ("No connections yet"). Clicking either the top-right "+ Add connection" button or the "Add connection" button inside the empty Local card both navigate to Settings → AI providers → Agent harnesses, instead of opening a form to add an MCP server (e.g. for Notion). Expected: an MCP server config form (name, command/URL, env vars). App version 0.6.2.

### Steps to reproduce

1. Launch Berd
2. Go to Settings → Connections
3. In the "Local" section, click "Add connection" — either the top-right "+ Add connection" button or the "Add connection" button inside the empty "No connections yet" card
4. Observe the result

### What you expected to happen

A form to configure a new MCP server (name, command or URL, args, environment variables) so I can connect an external service such as Notion.

### What actually happened

Both "Add connection" buttons on the Connections page redirect to Settings → AI providers → Agent harnesses instead — the page for connecting model providers to agent harnesses (Goose, Claude Code, Codex, etc.), which is unrelated to adding an MCP server. Reproduced twice, same result both times.

### How often does it happen?

Every time — reliably reproducible

### Berd version

0.6.2

### Operating system

macOS (Apple Silicon)

### Model and provider

n/a

### Relevant log output

```text
[2026-08-31][09:59:58][berd_lib::services::diagnostic_log][ERROR] [diagnostic] category=renderer event=unhandled_rejection app_version=0.6.2 platform=macos errorName=Error message="Command plugin:shell|open not allowed by ACL" stack="" windowKind=main

[2026-08-31][10:44:08][berd_lib::services::diagnostic_log][ERROR] [diagnostic] category=renderer event=unhandled_rejection app_version=0.6.2 platform=macos errorName=Error message="Command plugin:shell|open not allowed by ACL" stack="" windowKind=main

No other entries in the log reference the Connections/AI providers navigation. These "shell|open not allowed by ACL" errors are the only anomalies around the time I clicked Add connection — my guess (not confirmed) is the button tries to open something externally (e.g. an OAuth link) which the app's permissions block, and it silently falls back to the AI providers screen instead of showing an error.
```

### Screenshots, recordings, or other context

Start at the Settings → Connections page entry point and trace both Add connection buttons through their navigation handlers, then reproduce the behavior on the latest release. Done means both buttons open an MCP server configuration form with fields for name, command or URL, args, and environment variables, rather than the AI providers page.

Written by the indexing model from the issue text.