feat: add runbook studio dashboard and universal palette

[darwvin-dev]

Summary

• rebuild the operator experience directly on the hardened operator-data-plane head
• replace the empty workspace with a persisted fleet operations dashboard
• add Runbook Studio with bounded YAML import/export, validation and server-authoritative dry-run rendering
• force confirmation for destructive commands after variable rendering and execute only backend-prepared runbook steps
• add retry-from-failure while recalculating confirmation gates
• upgrade Ctrl+K into a universal palette indexing servers, health, diagnostics, runbooks, snippets, tabs and operator actions
• require explicit confirmation for snippet execution and call out production targets
• add cooperative multi-host cancellation between bounded batches
• load responsive operator-experience styling and route SFTP through the transfer queue

Safety

• no runbook command executes from client-only substitution
• malformed/unresolved variables block preparation
• destructive rendered commands cannot opt out of confirmation
• multi-host cancellation stops future batches while in-flight SSH commands finish and remain auditable

Merge gate

Retarget to main after the operator-data-plane PR merges. No helper workflow may remain. Merge only after exact-head PR title, Linux/macOS CI, security gates, Rust tests/build, frontend tests/build and live SSH integration are green.