fix: plasma-bigscreen build — correct source hash and add Qt6QmlPrivate dep

- Set correct source hash for fetchFromGitLab (was placeholder)
- Add qtdeclarative to build inputs for Qt6::QmlPrivate target
- Patch CMakeLists.txt to find_package(Qt6 QmlPrivate) before QCoro6,
  working around QCoro not importing its own transitive dependency

Verified: boxy, zix790prors, and nix-book all build successfully.

roles/plasma-bigscreen/package.nix

@@ -30,6 +30,7 @@
   libkscreen,
   kdeconnect-kde,
   # Qt 6
+  qtdeclarative,
   qtmultimedia,
   qtwebengine,
   # Other
@@ -50,7 +51,7 @@ mkKdeDerivation {
     repo = "plasma-bigscreen";
     rev = "bd143fea7e386bac1652b8150a3ed3d5ef7cf93c";
     # Build will fail with hash mismatch — copy the correct hash from the error
-    hash = "sha256-AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=";
+    hash = "sha256-y439IX7e0+XqxqFj/4+P5le0hA7DiwA+smDsD0UH/fI=";
   };
 
   extraNativeBuildInputs = [
@@ -80,6 +81,7 @@ mkKdeDerivation {
     libkscreen
     kdeconnect-kde
     # Qt
+    qtdeclarative
     qtmultimedia
     qtwebengine
     # Other
@@ -98,6 +100,12 @@ mkKdeDerivation {
     # against a stable Plasma release.
     substituteInPlace CMakeLists.txt \
       --replace-fail 'set(PROJECT_VERSION "6.5.80")' 'set(PROJECT_VERSION "${plasma-workspace.version}")'
+
+    # QCoro6Qml links against Qt6::QmlPrivate but doesn't import it.
+    # Add find_package for Qt6QmlPrivate before QCoro is loaded.
+    substituteInPlace CMakeLists.txt \
+      --replace-fail 'find_package(QCoro6' 'find_package(Qt6 ''${QT_MIN_VERSION} CONFIG REQUIRED COMPONENTS QmlPrivate)
+find_package(QCoro6'
   '';
 
   preFixup = ''