Enable maxLength on credential entry rows for Adw >= 1.6

[zugaldia]

Summary

• Adds isAdwVersionAtLeast(major, minor, micro) utility to AppUtils.kt for runtime libadwaita version checks
• Adds ADW_MAX_LENGTH_MIN_MAJOR_VERSION / ADW_MAX_LENGTH_MIN_MINOR_VERSION constants (1.6) to AppConstants.kt
• Conditionally sets maxLength on EntryRow / PasswordEntryRow in AddCredentialDialog only when Adw ≥ 1.6, working around a java-gi crash in earlier versions
• Refactors the existing startup Adw version check in App.kt to use the new helper

Software-side validation in validateInput() continues to enforce length limits on runtimes older than 1.6.

Test plan

• Run on Adw ≥ 1.6: verify credential name and API key fields enforce the character limit visually
• Run on Adw < 1.6: verify no crash, fields accept input freely, and overlong submissions are still rejected by validation
• Confirm ./gradlew detekt passes cleanly

[zugaldia]

Refs: jwharm/java-gi#335.