feat(client): HelloWorld, the reference application

Every other test of the plugin architecture reads source for the *absence* of
app names. That proves nobody wrote a special case for Videos; it cannot prove
a genuinely new application works, because there was no new application.

This is one. It stores directories, appears as a tab, has a settings pane and
lists files, and the node has never heard its name outside a single allow-list
entry. Two files and one registry line, which is the claim
`docs/refactor-groups.md` §4.1 makes.

It ships hidden behind `?dev=1` (`dev: true` in the registry, the same opt-in
shape as transport.js's `?trace=1`). Registering it normally would put a toy
app in every operator's group; not registering it would prove nothing, since
registration is exactly what is claimed to be sufficient.

**Adding it found two places where the claim was nearly true rather than true,
and both are fixed by making the code less app-specific:**

`group-settings.js` fell back to the whole registry when a group had no
`enabled_apps` yet — which would have turned a hidden app on for everyone. It
asks `availableApps()` now.

`group-page.js` wrote out `videoDirectories` / `musicDirectories` /
`photoDirectories` by hand, so a fifth app would have needed that file edited.
It derives `<key>Directories` from the registry.

Neither was found by reading; both were found by adding the app, which is the
whole reason it exists.

Verified in a real Electron window as well as by the tests: hidden by default,
present with the flag, offered its own settings section, and listing exactly
the files under its configured folder and its subfolders — not the ones beside
it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011pvMdvLBG92jyhvD5pD6us

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