Fix CI: bump PHP requirement to ^8.4 (Symfony 8 enforces it)

CI was failing on the Install-bundle-dependencies step because
shivammathur/setup-php was installing 8.3 while Symfony 8.x dependencies
declare php >= 8.4. Local composer install worked because the dev box
runs PHP 8.5.5; CI doesn't.

Bumps:
  - framework/php/composer.json
  - framework/skeleton/symfony/composer.json
  - examples/todo/symfony/composer.json
  - .gitea/workflows/ci.yml         php-version: '8.3' → '8.4'
  - .gitea/workflows/release.yml    same
  - PLAN.md §13 Phase 1 *Detailed scope* PHP minimum row

PHPStan / cs-fixer / PHPUnit stay green locally.

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

framework/php/composer.json

@@ -4,7 +4,7 @@
     "type": "symfony-bundle",
     "license": "proprietary",
     "require": {
-        "php": "^8.3",
+        "php": "^8.4",
         "symfony/framework-bundle": "^8.0",
         "symfony/mercure": "^0.7",
         "symfony/security-bundle": "^8.0",

framework/qml/src/BackendConnection.h

@@ -67,6 +67,17 @@ public:
     /// hits Retry on the Offline overlay). Dev mode: re-probe.
     Q_INVOKABLE void restart();
 
+    /// Bundled mode: ask the bundled `AppImageUpdate.AppImage` sidecar
+    /// whether a newer release is available. Dev mode: emits
+    /// `updateCheckFailed` with an explanatory string. Result is signalled
+    /// via updatesAvailable / noUpdatesAvailable / updateCheckFailed.
+    Q_INVOKABLE void checkForUpdates();
+
+    /// Bundled mode: invoke the sidecar to download and apply the
+    /// available update in place. The user has to restart the app
+    /// after this completes; emits `updateApplied` / `updateApplyFailed`.
+    Q_INVOKABLE void applyUpdate();
+
 signals:
     void urlChanged();
     void tokenChanged();
@@ -77,6 +88,12 @@ signals:
     /// MercureClient pick the new value up on next request (§3 *Edge cases*).
     void tokenRotated(const QString& newToken);
 
+    void updatesAvailable();
+    void noUpdatesAvailable();
+    void updateCheckFailed(const QString& reason);
+    void updateApplied();
+    void updateApplyFailed(const QString& reason);
+
 private slots:
     void probe();
     void onProbeFinished();

framework/skeleton/symfony/composer.json

@@ -3,7 +3,7 @@
     "license": "proprietary",
     "minimum-stability": "stable",
     "require": {
-        "php": "^8.3",
+        "php": "^8.4",
         "symfony/framework-bundle": "^8.0",
         "symfony/runtime": "^8.0",
         "symfony/dotenv": "^8.0",