cold-wallet: bump build to 1.2.0+18

[illuzen]

The APK release workflow tags cold-wallet-v<pubspec version> and refuses an existing tag; cold-wallet-v1.2.0+17 was already released before #677 merged, so the build for the NEAR cold signing release failed at the tagging step.

Bumps cold-wallet-app/pubspec.yaml to 1.2.0+18. Nothing else changes.

[n13]

Reviewer model: GPT-6 Sol

Verdict: COMMENT (retrospective approval). The PR merged during review; I found no blocking issues in the version bump.

cold-wallet-app/pubspec.yaml:5 changes only the cold wallet version from 1.2.0+17 to 1.2.0+18. The APK workflow reads that value and constructs cold-wallet-v1.2.0+18; the previous +17 tag exists on the remote, while the new tag is available.

Validation: inspected the complete base-to-head diff and release workflow; git diff --check passed; confirmed the workflow-extracted version and remote tag availability. No Flutter build or tests were run for this metadata-only change. The Analyze CI check was still in progress at review time.