🛡️ Sentinel: [HIGH] Fix OAuth CSRF Vulnerability

[LeanBitLab]

🚨 Severity: HIGH
💡 Vulnerability: The OAuth flow used a predictable System.currentTimeMillis() for the state parameter and failed to verify it during the callback. This left the authentication flow vulnerable to Cross-Site Request Forgery (CSRF).
🎯 Impact: An attacker could potentially associate an arbitrary Reddit account or manipulate the login session of the victim, gaining unauthorized access or executing actions on behalf of the user.
🔧 Fix:

• Modified launchLogin to generate a secure random string using UUID.randomUUID().toString().
• The generated state is now stored in SharedPreferences prior to login.
• In handleOAuthCallback, the state from the callback URI is strictly verified against the stored state, and the stored state is immediately removed to ensure single use.
  ✅ Verification:
• The application compiles and tests pass successfully via ./gradlew test.
• Logging in via Reddit will function exactly as before, but the state variable logged or sent will now be a random UUID, and the callback verification prevents execution if mismatched.

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PR created automatically by Jules for task 9581275075405251945 started by @LeanBitLab

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