Inline chat images outside $HOME fail with asset protocol not configured

### 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

#166 is the nearest report. That one is the click-to-open / `opener` allowlist failure (`Not allowed to open path`). This issue is the separate inline-preview failure: the same local image is refused by the Tauri asset protocol before it can render in chat.

### What's broken

Local images inside an attached project folder do not render inline in chat when that folder is outside `$HOME`. Berd logs `asset protocol not configured to allow the path` and the image stays broken, even though the file exists, is readable, and is inside the session cwd.

### Steps to reproduce

1. Launch Berd 0.6.2 on macOS.
2. Open a project whose working directory is outside `$HOME`. The failing path here was an attached project under `/Users/Shared/...`.
3. Confirm the folder is the session cwd.
4. In that chat, have the agent emit a normal markdown image pointing at a real PNG/JPEG inside the attached folder, for example `` or a relative path that resolves to the same file.
5. Look at the rendered message.

Model/provider does not appear to matter. Reproduced with existing project images that the agent and the shell can both read.

### What you expected to happen

Because the image is a local file inside the attached project / session cwd, Berd should render it inline the same way it does for images under `$HOME`.

`MarkdownImage` already resolves local markdown images against the session cwd and then calls `convertFileSrc(..., "asset")`. That path should be allowed for attached project folders, not only `$HOME` and Berd's own media caches.

### What actually happened

The image does not render inline. `berd.log` records:

`[tauri::protocol::asset][ERROR] asset protocol not configured to allow the path: /Users/Shared/.../file.png`

This is not a missing file, a bad markdown path, or a macOS permission problem:

- The same files exist and are readable.
- `/usr/bin/open` opens them.
- The project folder is attached as cwd.
- The failure is Tauri refusing the `asset:` protocol for a path outside the compiled scope.

`src-tauri/tauri.conf.json` on v0.6.2 enables `assetProtocol` with this scope only:

- `$APPDATA/avatars/media/**/*`
- `$APPDATA/artifacts/media/**/*`
- `$APPDATA/project-artifacts/media/**/*`
- `$APPDATA/home-widget-media/**/*`
- `$HOME/**/*`
- `$TEMP/**/*`

An attached project under `/Users/Shared` is outside every one of those globs, so inline preview is denied by configuration.

### How often does it happen?

Every time — reliably reproducible

### Berd version

0.6.2

### Operating system

macOS (Apple Silicon)

### Model and provider

n/a for the render itself. Observed in a Goose chat when the transcript included local markdown images from the attached project.

### Relevant log output

```text
[2026-08-21][01:48:12][tauri::protocol::asset][ERROR] asset protocol not configured to allow the path: /Users/Shared/PROJECTS_shared/LALA/content-lab/LANGUAGE/indonesian/production-pipeline/2.keyframes/production-readiness/family-world-packets/ID-WORLD-ID-RESTAURANT-PARMESAN-01/environment-layout/v1/review/preview/clean-plan.png
[2026-08-21][23:26:03][tauri::protocol::asset][ERROR] asset protocol not configured to allow the path: /Users/Shared/PROJECTS_shared/LALA/content-lab/LANGUAGE/indonesian/production-pipeline/2.keyframes/revision-evidence/2026-08-21-family-occupied-hero-first/heroes/03-banyak/ID-FN-030--ms01/candidate-b.png
```

The same class of path failed repeatedly in this log (41 `asset protocol not configured` lines). `/usr/bin/open` on the same machine opens those files successfully.

### Screenshots, recordings, or other context

`src/features/chat/ui/MarkdownImage.tsx` routes local markdown images through `convertFileSrc(resolvedPath, "asset")` after checking that the resolved path is within the session cwd. The chat UI is doing the intended thing; the asset-protocol allowlist then rejects the file because attached project roots outside `$HOME` are not in scope.

Related but separate: clicking the same path shows `Not allowed to open path` (#166). That is the opener plugin allowlist. This issue is only the inline preview / asset-protocol deny.

Workaround that works today: copy or symlink the image into `$HOME` (for example `~/Downloads`) and link that copy instead.

Start with src/features/chat/ui/MarkdownImage.tsx and src-tauri/tauri.conf.json, then reproduce with a markdown image in an attached project outside $HOME. Check that the asset protocol scope covers the session cwd without weakening unrelated restrictions. Done means the image renders inline and the asset-protocol denial no longer appears for an allowed project path.

Written by the indexing model from the issue text.