Fix #18: Exception leak remediation across 87 PHP files

- Replaced all client-facing $e->getMessage() with generic error messages
- Added error_log() with filename prefix to all catch blocks
- Covered jsonError(), echo, and json_encode() response patterns
- Also fixed 2 remaining display_errors=1 and add_invoice.php leak
- Script-assisted fix for 75 files, manual fix for 12 remaining edge cases

backend/Admin/auth/approve_admin.php

@@ -44,5 +44,5 @@ try {
 
 } catch (Exception $e) {
     error_log("[Approve Admin Error] " . $e->getMessage());
-    jsonError("Server Error: " . $e->getMessage());
+    jsonError("An internal error occurred. Please try again later.");
 }