feat(dashboard): usage cost display + create-sequence template picker

[ovasylenko]

Summary

The dashboard halves of the cost (#56) and template-gallery (#57) features. Neither a Usage view nor a sequence-creation flow existed in the dashboard — both built fresh following the house conventions (PageHeader/Section/Metric/Table kit, usePolling + api.ts helpers, node:test suites).

• /usage page: "Estimated spend" headline (total_cost_usd, labeled "USD · est." with list-price tooltip), token/event totals, per-(kind, model) table with right-aligned estimated-cost column; unknown models show an em-dash with an explanatory tooltip. New fmtUsd helper (2 decimals ≥ $1, 4 below).
• Create-from-template picker: "New sequence" flow on the Sequences page with a template card grid — blank + the 5 CLI templates (default scaffold mirrored from orch8-cli/src/templates.rs, pattern JSONs inlined from docs/agent-patterns/), pre-filling the JSON editor with tenant/namespace adaptation. A drift-guard test deep-equals each embedded template against the actual docs/agent-patterns/*.json files.
• Added typecheck and test scripts to dashboard/package.json.

Tests

74/74 pass (node --test), tsc -b clean, production vite build compiles.

Notes

• CI has no dashboard job today — the new scripts are ready for one (follow-up).
• Dashboard's package-lock.json is stale (predates this change); pnpm with the workspace lockfile is the working install path.

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