Simplify validation, compression heuristics, and notification scheduling (#841)

* Simplify validation, compression heuristics, and notification scheduling

* Consolidate notification logic and add InputValidator monitoring

Follow-up improvements to address PR feedback:

1. Consolidate notification functions
   - Add interruptionLevel parameter to sendLocalNotification
   - Refactor sendNetworkAvailableNotification to use consolidated function
   - Removes 12 lines of duplicate code

2. Add monitoring to InputValidator
   - Log control character rejections for production monitoring
   - Privacy-preserving: logs length + count, not actual content
   - Uses .security category for proper log routing

3. Add comprehensive InputValidator tests
   - 28 test cases covering validation, control characters, unicode, edge cases
   - Ensures behavioral changes are well-tested and documented

---------

Co-authored-by: jack <jackjackbits@users.noreply.github.com>

bitchat/Utils/CompressionUtil.swift

@@ -61,15 +61,13 @@ struct CompressionUtil {
         // 1. Data is too small
         // 2. Data appears to be already compressed (high entropy)
         guard data.count >= compressionThreshold else { return false }
-        
-        // Simple entropy check - count unique bytes
-        var byteFrequency = [UInt8: Int]()
-        for byte in data {
-            byteFrequency[byte, default: 0] += 1
-        }
-        
-        // If we have very high byte diversity, data is likely already compressed
-        let uniqueByteRatio = Double(byteFrequency.count) / Double(min(data.count, 256))
+
+        // Quick uniqueness check — a high diversity of bytes usually means the
+        // payload is already compressed. We only need to know how many unique
+        // values exist rather than keeping full frequency counts.
+        let uniqueByteCount = Set(data).count
+        let sampleSize = min(data.count, 256)
+        let uniqueByteRatio = Double(uniqueByteCount) / Double(sampleSize)
         return uniqueByteRatio < 0.9 // Compress if less than 90% unique bytes
     }
 }

bitchat/Utils/InputValidator.swift

@@ -1,4 +1,5 @@
 import Foundation
+import BitLogger
 
 /// Comprehensive input validation for BitChat protocol
 /// Prevents injection attacks, buffer overflows, and malformed data
@@ -16,29 +17,28 @@ struct InputValidator {
     // MARK: - String Content Validation
     
     /// Validates and sanitizes user-provided strings used in UI
+    ///
+    /// Rejects strings containing control characters to prevent potential security issues
+    /// and UI rendering problems. This strict approach ensures data integrity at input time.
     static func validateUserString(_ string: String, maxLength: Int) -> String? {
-        // Check empty
-        guard !string.isEmpty else { return nil }
-
-        // Trim whitespace
         let trimmed = string.trimmingCharacters(in: .whitespacesAndNewlines)
         guard !trimmed.isEmpty else { return nil }
-
-        // Check length
         guard trimmed.count <= maxLength else { return nil }
 
-        // Remove control characters
+        // Reject control characters outright instead of rewriting the string.
+        // This prevents injection attacks and ensures consistent UI rendering.
         let controlChars = CharacterSet.controlCharacters
-        let cleaned = trimmed.components(separatedBy: controlChars).joined()
-        
-        // Ensure valid UTF-8 (should already be, but double-check)
-        guard cleaned.data(using: .utf8) != nil else { return nil }
-        
-        // Prevent zero-width characters and other invisible unicode
-        let invisibleChars = CharacterSet(charactersIn: "\u{200B}\u{200C}\u{200D}\u{FEFF}")
-        let visible = cleaned.components(separatedBy: invisibleChars).joined()
-        
-        return visible.isEmpty ? nil : visible
+        if !trimmed.unicodeScalars.allSatisfy({ !controlChars.contains($0) }) {
+            // Log rejection for monitoring, without exposing actual content for privacy
+            let controlCharCount = trimmed.unicodeScalars.filter { controlChars.contains($0) }.count
+            SecureLogger.debug(
+                "Input validation rejected string (length: \(trimmed.count), control chars: \(controlCharCount))",
+                category: .security
+            )
+            return nil
+        }
+
+        return trimmed
     }
     
     /// Validates nickname