Add global account selection controls

[yvette-carlisle]

Summary

• Add a global Codex account fixed-selector backed by the Decodex config and dashboard control actions.
• Keep account selection in the Accounts panel with the double-click confirmation interaction and fixed/balanced mode meta.
• Tighten dashboard placeholders, account ordering, mock websocket controls, docs, and status tests.

Verification

• git diff --check
• node --check dev/operator-dashboard-mock.mjs
• dashboard inline script parse check
• cargo make fmt-check
• cargo make check-rust
• cargo test -p decodex