Use compact account fallback placeholders

[yvette-carlisle]

Summary

• Render missing account plan, usage, reset, and credits data with - instead of long fallback labels
• Keep accessibility labels explicit with unavailable wording
• Update dashboard string assertions for the new compact placeholders

Verification

• cargo test -p decodex operator_dashboard_accounts
• swift build --package-path apps/decodex-app
• cargo make fmt-rust-check
• git diff --check
• semantic drift helper: no docs changes or stale phrase hits