feat(#506): add Import example dashboard to the File menu

[BorisTyshkevich]

What & why

Adds File ▾ → Import example dashboard… — a modal that lists the three
shipped flagship example dashboards (ClickHouse Operations, Shop Charts,
OnTime Charts) from a checked-in manifest, and imports the selected one
through the existing Dashboard import/codec/validation/dependency-closure/
commit pipeline. It's purely additive (same rule as the existing Import
dashboard… row): an existing Dashboard is never replaced or merged into,
and Cancel/Escape/an outside click leave the workspace untouched.

The catalogue (examples/dashboard-manifest.json + the three referenced
examples/*.json files) is compiled at build time by the new
build/compile-example-dashboards.mjs into a checked-in generated module
(src/generated/example-dashboards.ts), mirroring the existing JSON-Schema
generation pipeline (build/compile-json-schemas.mjs) — so it ships embedded
in the single-file production artifact rather than as separate served files.
npm run check:examples (wired into prebuild/pretest, alongside
check:schemas) fails the build on a missing, malformed, or stale example so
one can't ship unnoticed.

Closes #506

Checklist

• npm test passes (the per-file coverage gate is non-negotiable)
• Tests added/updated in the same change as the code
• npm run build succeeds (single-file dist/sql.html)
• Layers kept honest: pure logic in src/core/, network in src/net/ (injected fetch), DOM in src/ui/
• No new runtime dependency
• README / CHANGELOG.md ([Unreleased]) updated
• Reconciled affected tracked work — Add example Dashboard import from the File menu #506 isn't tracked in roadmap Roadmap to 1.0.0 #68 and its "Related" issues (Make Dashboard a first-class module with one-dashboard-per-current-workspace v1 UX #280, Replace curated Dashboard filters with inferred Variables and batched option queries #447) are already closed/shipped, so nothing there needed re-scoping

Test plan

• npm test — 5961 unit tests, 100%/100% statements/lines per-file coverage gate green
• npm run pretest — schemas, examples, arch boundaries, tsc --noEmit all green
• npm run build — artifact builds (~2 MiB raw / ~585 KiB gzip)
• Full Playwright e2e suite (chromium/firefox/webkit, --workers=1) — 481 passed, 2 skipped (unrelated), 0 failed, including the new tests/e2e/import-example-dashboard.spec.js (first-ever File-menu e2e spec: dialog listing/selection, Cancel-no-change, Escape-no-change, Import-appends-and-opens)
• Independent read-only review pass over the full diff — no correctness bugs found; one docs nit fixed (README example order) and one non-blocking a11y follow-up filed as a11y: example-dashboard radiogroup lacks roving tabindex + arrow-key navigation #509 (radiogroup keyboard pattern)