bug(dev): `just dev` leaves non-executable sidecar stubs, breaking ACP authentication

## Summary

A source build launched with `just dev` can leave the desktop sidecar helpers as zero-byte, non-executable files. The desktop app then fails when it invokes `buzz-acp` for Claude Code sign-in discovery:

```
Couldn't load sign-in options: failed to run buzz-acp auth helper: Permission denied (os error 13)
```

This also prevents other desktop-managed agent features that require the bundled sidecars.

## Reproduction

1. Clone the repository on macOS (Apple Silicon).
2. Run the documented setup, then start the app with `just dev`.
3. In the desktop app, open the Claude Code integration and attempt to load sign-in options.

The files under `desktop/src-tauri/binaries/` are placeholders created by `_ensure-sidecar-stubs` and remain zero bytes / mode `0644`.

## Expected

`just dev` should make executable `buzz-acp`, `buzz-agent`, `buzz-dev-mcp`, `buzz`, and `git-credential-nostr` sidecars available to the Tauri desktop process.

## Actual

The Tauri app attempts to execute the `buzz-acp` helper and macOS returns `Permission denied (os error 13)`.

## Likely cause

`_ensure-sidecar-stubs` intentionally creates placeholder binaries for Tauri compile-time validation, but the `dev` recipe does not copy the compiled helpers into `desktop/src-tauri/binaries/` afterward. `desktop-standalone` already performs that copy, so the behavior is inconsistent.

A local workaround was to build the helper binaries and copy them into `desktop/src-tauri/binaries/-aarch64-apple-darwin`; after that, `buzz-acp auth-methods --json` returned the Claude sign-in methods successfully.

## Suggested fix

Reuse the copy-and-`chmod +x` block from `desktop-standalone` in `just dev` after its `cargo build` step, or factor it into a shared recipe used by both launch paths.

Start with the just dev recipe, _ensure-sidecar-stubs, and the desktop-standalone recipe, then compare how each handles the cargo build and copies files into desktop/src-tauri/binaries/. Verify that just dev produces executable sidecars for the listed names and run buzz-acp auth-methods --json through the desktop setup to confirm the helper can run.

Written by the indexing model from the issue text.