chore(github): complete issue/PR templates and wire in-app bug report to them

Add a feature request template and a config.yml (blank issues disabled,
security reports routed to SECURITY.md) alongside the existing bug report
template, plus a PR template with a checklist matching the project's
security rules. Point the app's "Report a bug" button at the new GitHub
template now that the repo is hosted there instead of the old self-hosted
tracker.

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,26 @@
+## Summary
+
+<!-- What does this PR change and why? -->
+
+## Related issue
+
+<!-- Closes #... (if applicable) -->
+
+## Type of change
+
+- [ ] Bug fix
+- [ ] New feature
+- [ ] Refactor / cleanup
+- [ ] Documentation
+- [ ] Build / CI
+
+## Testing
+
+<!-- How was this verified? e.g. `dotnet test`, manual run on Windows/Linux/Android, golden-vector comparison -->
+
+## Checklist
+
+- [ ] `dotnet test` passes
+- [ ] No seed/private key material logged or persisted in plaintext (if touching Crypto/Wallet/Storage)
+- [ ] Server responses still validated with Merkle + checkpoints (if touching Spv/Net)
+- [ ] Code/comments/commit messages are in English