chore(android): lock the app to portrait orientation

[netqo]

Summary

Pins MainActivity with android:screenOrientation="portrait" so the app no longer rotates into landscape.

Rationale

Every screen is designed and laid out for a portrait phone; landscape was never a supported layout. Locking the launcher (single) activity is the simplest, global way to enforce it.

Verification

• ./gradlew assembleDebug -> BUILD SUCCESSFUL
• On device: rotating the phone no longer rotates the app; it stays portrait.

Test plan

• Debug APK assembles
• Manual device check: app stays portrait when the phone is rotated

Checklist

• Commit messages follow Conventional Commits (see CONTRIBUTING.md).
• Branch name follows the naming convention (chore/).
• The change is scoped: manifest only.
• If user-visible behavior changed, CHANGELOG.md has been updated under [Unreleased]. (One-line manifest config; left out of the changelog to avoid a churn conflict with the in-flight feature PR.)
• If a new dependency was added, it is justified in the PR description. (No new dependency.)