docs: consolidate QR code implementation documentation

Merge multiple QR code documentation files into a single comprehensive guide
that accurately reflects the current implementation. The consolidated guide:

- Combines information from qr-code-implementation-guide.mdc,
  qr-code-handling-rule.mdc, and camera-implementation.md
- Clarifies the relationship between ContactQRScanView and ContactQRScanShowView
- Streamlines build configuration documentation
- Adds detailed sections on error handling, security, and best practices
- Improves organization and readability of implementation details
- Removes redundant information while preserving critical details

This change improves documentation maintainability and provides a single
source of truth for QR code implementation details.

src/views/ContactQRScanShowView.vue

@@ -326,29 +326,29 @@ export default class ContactQRScanShow extends Vue {
           switch (state) {
             case 'in_use':
               this.error = "Camera is in use by another application";
-              this.isScanning = false;
-              this.$notify(
-                {
-                  group: "alert",
-                  type: "warning",
+        this.isScanning = false;
+        this.$notify(
+          {
+            group: "alert",
+            type: "warning",
                   title: "Camera in Use",
                   text: "Please close other applications using the camera and try again",
-                },
-                5000,
-              );
+          },
+          5000,
+        );
               break;
             case 'permission_denied':
-              this.error = "Camera permission denied";
-              this.isScanning = false;
-              this.$notify(
-                {
-                  group: "alert",
-                  type: "warning",
-                  title: "Camera Access Required",
+          this.error = "Camera permission denied";
+          this.isScanning = false;
+          this.$notify(
+            {
+              group: "alert",
+              type: "warning",
+              title: "Camera Access Required",
                   text: "Please grant camera permission to scan QR codes",
-                },
-                5000,
-              );
+            },
+            5000,
+          );
               break;
             case 'not_found':
               this.error = "No camera found";