Security fixes and improvements (#374)

- Fix force unwrapping in NostrIdentity bech32 functions that could crash on non-ASCII input
- Add comprehensive input validation for all protocol messages (peer IDs, nicknames, timestamps)
- Strengthen keychain security with better sandbox detection and consistent app group usage
- Implement secure memory clearing for cryptographic keys and shared secrets
- Fix panic mode not reconnecting to mesh by restarting services after emergency disconnect

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

bitchat/Nostr/NostrIdentity.swift

@@ -165,6 +165,14 @@ enum Bech32 {
         }
         
         let hrp = String(bech32String[..<separatorIndex])
+        
+        // Validate HRP contains only ASCII characters
+        for char in hrp {
+            guard char.asciiValue != nil else {
+                throw Bech32Error.invalidCharacter
+            }
+        }
+        
         let dataString = String(bech32String[bech32String.index(after: separatorIndex)...])
         
         // Convert characters to values
@@ -238,11 +246,17 @@ enum Bech32 {
     private static func hrpExpand(_ hrp: String) -> [UInt8] {
         var result = [UInt8]()
         for c in hrp {
-            result.append(UInt8(c.asciiValue! >> 5))
+            guard let asciiValue = c.asciiValue else {
+                return [] // Return empty array for invalid input
+            }
+            result.append(UInt8(asciiValue >> 5))
         }
         result.append(0)
         for c in hrp {
-            result.append(UInt8(c.asciiValue! & 31))
+            guard let asciiValue = c.asciiValue else {
+                return [] // Return empty array for invalid input
+            }
+            result.append(UInt8(asciiValue & 31))
         }
         return result
     }