Make mvnw executable

[devin-ai-integration]

Summary

mvnw was committed without the executable bit, so ./mvnw fails with Permission denied on a fresh clone (and sh mvnw also fails, since the wrapper resolves .mvn/wrapper/maven-wrapper.properties relative to $0's directory and picks up mvnw/.mvn/...). Sets mode 100644 -> 100755; no content or behavior change.

Verified afterwards: ./mvnw clean package -DskipTests builds, and the app runs against MySQL 8 on localhost:3306 serving http://localhost:8080 (register / login / deposit / withdraw / transaction history all work).

Link to Devin session: https://app.devin.ai/sessions/e87cf6340ffa4e689bb20d099eed3c2c
Requested by: @patrickbradley-cog

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