v0.4: licensing — server validation, DPAPI cache, Ed25519 signed responses

UX bonus: clicking "Reprendre" on an interrupted download skips the
release-notes confirmation dialog (the user already approved when they
first clicked Installer).

Server side
-----------
- migrations/001_init.sql: licenses, license_machines, rate_limit,
  audit_log on InnoDB/utf8mb4. Foreign keys, unique on license_key, slot
  uniqueness per (license_id, machine_id).
- api/lib/Db.php: thin PDO singleton (exception mode, prepared, no emul).
- api/lib/Crypto.php: Ed25519 sign/verify via libsodium (sodium_crypto_*),
  HMAC-SHA-256 helper for v0.6, canonicalJson() that strips `signature`
  before serializing — must match exactly the encoding done client-side
  before verify.
- api/routes/ValidateLicense.php: POST /license/validate. Looks up the
  key, walks the machine slot logic (insert or update last_seen),
  enforces max_machines, returns a payload signed Ed25519 + status of
  valid/expired/revoked/machine_limit_exceeded/invalid. Audit logs every
  outcome. Rate-limit 10/min/IP via the rate_limit table.
- tools/generate-keypair.php: prints a fresh sodium keypair so the
  operator drops the hex into config.php and the public_key_hex into the
  launcher resource.
- tools/issue-license.php: PRSRV-XXXX-XXXX-XXXX-XXXX generator (32-char
  unambiguous alphabet), inserts the license, prints the key once.
- tools/sign-manifest.php: now also signs the manifest itself with
  Ed25519 after computing the per-zip sha256s.
- config.example.php: schema rewritten with sections db / hmac / ed25519
  / jwt / rate-limit. config.php remains gitignored.

Client side
-----------
- Models/License.cs: LicenseValidationRequest + LicenseValidationResponse
  with CanDownload(VersionManifest) — entitlement_until vs version's
  minLicenseDate. The status valid|expired|revoked|machine_limit_exceeded
  flow is preserved end-to-end.
- Core/Licensing/LicenseService.cs:
  * machineId = SHA-256 of HKLM/Software/Microsoft/Cryptography/MachineGuid
    + UserName (stable, no PII leak)
  * online ValidateAsync calls /license/validate with launcher version
  * embedded server-pubkey.txt drives Ed25519 verification of the
    response (skipped gracefully if pubkey not yet provisioned)
  * SaveCached / GetCached use DPAPI CurrentUser scope on the license
    key; the cleartext key never touches disk
  * GetCached has a 7-day offline grace window after the last successful
    validation, so going offline doesn't lock the user out
- Core/Resources/server-pubkey.txt: EmbeddedResource. Default content is
  a comment, which the service treats as "no pubkey configured" and
  bypasses verification. Operator pastes the real hex post-deploy and
  rebuilds.
- Core/PSLauncher.Core.csproj: Polly, NSec.Cryptography (Ed25519),
  System.Security.Cryptography.ProtectedData (DPAPI).
- App/Views/OnboardingDialog.xaml(.cs): first-launch / "🔑 Activer"
  modal. Calls LicenseService, displays status messages with red
  foreground on errors and green-tinted secondary text otherwise.
- ViewModels/VersionRowViewModel.cs: new LicenseAllowsDownload property.
  Install button label switches to "🔒 License insuffisante" when the
  user's entitlement_until precedes the version's minLicenseDate;
  CanInstall is false in that case so the click is a no-op too.
- ViewModels/MainViewModel.cs: loads the cached license at startup (no
  network call), surfaces it as LicenseSummary in the top bar, exposes
  ActivateLicenseCommand to (re)open the onboarding dialog. RebuildList
  applies the per-version license filter so older installed versions
  remain launchable but newer-than-license ones can't be downloaded.
- Views/MainWindow.xaml: top bar gains a "🔑 Activer / changer" button
  next to the license summary.

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

src/PSLauncher.App/ViewModels/VersionRowViewModel.cs

@@ -42,12 +42,20 @@ public sealed partial class VersionRowViewModel : ObservableObject
     [NotifyPropertyChangedFor(nameof(HasResumableDownload))]
     private long _resumableBytes;
 
+    [ObservableProperty]
+    [NotifyPropertyChangedFor(nameof(InstallButtonLabel))]
+    [NotifyPropertyChangedFor(nameof(LicenseAllowsInstall))]
+    [NotifyCanExecuteChangedFor(nameof(InstallCommand))]
+    private bool _licenseAllowsDownload = true;
+
     public bool HasResumableDownload => ResumableBytes > 0;
+    public bool LicenseAllowsInstall => LicenseAllowsDownload;
 
     public string InstallButtonLabel
     {
         get
         {
+            if (!LicenseAllowsDownload) return "🔒  License insuffisante";
             if (!HasResumableDownload) return "⬇  Installer";
             if (Remote is null || Remote.Download.SizeBytes <= 0) return "↻  Reprendre";
             var pct = (double)ResumableBytes / Remote.Download.SizeBytes * 100.0;
@@ -133,7 +141,7 @@ public sealed partial class VersionRowViewModel : ObservableObject
 
     [RelayCommand(CanExecute = nameof(CanInstall))]
     private void Install() => InstallHandler?.Invoke(this);
-    private bool CanInstall() => State == VersionRowState.AvailableIdle;
+    private bool CanInstall() => State == VersionRowState.AvailableIdle && LicenseAllowsDownload;
 
     [RelayCommand(CanExecute = nameof(CanUninstall))]
     private void Uninstall() => UninstallHandler?.Invoke(this);