Weight Codex account pool usage by capacity

[yvette-carlisle]

Summary

• weight Codex account usage estimates by account capacity multiplier
• show 20x / 1x weights in Decodex App and operator dashboard instead of plan names
• persist non-secret capacity weights in local usage history and document the display contract

Validation

• cargo make fmt-check
• cargo test -p decodex accounts::tests::
• cargo test -p decodex operator_dashboard_accounts
• cargo test -p decodex operator_dashboard_renders_account_usage_controls
• cargo test -p decodex operator_state_endpoint_serves_account_api_snapshot
• cargo make check-rust
• cargo make check-vstyle-rust
• swift test (apps/decodex-app)
• semantic drift audit: pass, no stale phrase hits
• Decodex App build/run verify