backoffice: dedicated Launcher page, scope-aware Sync

The launcher and the game versions are different release cadences
managed by the same operator. Mixing them on one page muddied the
workflow. Split them into two top-level nav entries.

SignManifest::run(string $scope)
--------------------------------
- 'all' (default, used by CLI tools/sign-manifest.php) — unchanged.
- 'versions' — only re-hashes the Proserve ZIPs and bumps `latest`.
- 'launcher' — only re-hashes the launcher EXE.
The Ed25519 sign step always runs at the end so the manifest stays
verifiable. Selectively hashing avoids unrelated noise (e.g. mass-hash
14 GB ZIPs when all you wanted was to update the launcher exe).

admin/launcher.php (new page)
-----------------------------
Self-contained page with the launcher state, Set/Remove forms, the
blue "🔁 Hasher le launcher + signer" button, and a list of the .exe
files present in builds/launcher/. Workflow doc inline.

admin/versions.php
------------------
Cleaned up: launcher card and its set_launcher / remove_launcher /
sync_launcher actions removed. The remaining global Sync button is
relabeled and now triggers scope='versions' (only Proserve ZIPs).

Layout::navHtml gains a "Launcher" item between Versions and Audit.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

server/admin/lib/Layout.php

@@ -38,6 +38,7 @@ HTML;
             'dashboard' => ['index.php',    'Dashboard'],
             'licenses'  => ['licenses.php', 'Licenses'],
             'versions'  => ['versions.php', 'Versions'],
+            'launcher'  => ['launcher.php', 'Launcher'],
             'audit'     => ['audit.php',    'Audit'],
         ];
         $links = '';