build: CI and the snap build with Flutter v3.47.8, as the laptop does

A patch release; the release tool refuses to build with another Flutter
than CI tests with.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

snap/snapcraft.yaml

@@ -83,7 +83,7 @@ parts:
   flutter:
     plugin: nil
     source: https://github.com/flutter/flutter.git
-    source-tag: '3.47.5'
+    source-tag: '3.47.8'
     source-depth: 1
     build-packages:
       - clang