chore(runner): shared e2e helpers, and a failure report that isn't empty (DEV-2203)

[danielzytohoc]

First of the DEV-2203 series (E2E suite for the demo runner, run against production).

What

• runner/e2e/helpers.ts — the suite's most-copied code becomes a shared module: stubShell and signIn existed verbatim in nine specs, the session-cleanup pattern in two, the console-noise filter in one. Also exports the readiness/grid helpers (previewReady, expectGridRendered) built on the documented data-preview-status test contract, and workspaceFiles() for the window.__HOT_FILES__ hook that lands with the Style-panel spec PR. New specs import from here; existing specs keep their local copies and migrate opportunistically — a mass rewrite would churn thousands of spec lines for zero behaviour change.
• Reporter fix — in CI the github reporter never writes playwright-report/, so the on-failure artifact both workflows upload has been empty since they were added. CI now also runs the html reporter, and the workflows upload test-results/ too (where traces actually land).
• fflate root devDependency — so upcoming specs can assert on the contents of a downloaded workspace zip (the app already builds those zips with fflate at the same version).

Verification

pnpm build + pnpm typecheck + pnpm test (360 pass) + full deterministic pnpm e2e (152 passed, 124 live-gated skips, 0 failures).

Part of DEV-2203; next in the series: share-view, editor/version, engine/docs-frameworks, style-panel specs, then the workflow wiring.

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Note

Low Risk
Test infrastructure and CI artifact wiring only; no production app or auth logic changes in this diff.

Overview
Introduces runner/e2e/helpers.ts as the shared Playwright toolkit for DEV-2203: deterministic shell stubs (stubShell, signIn), editor/preview helpers (activeEditor, workspaceFiles, pickFromMenu, previewReady, expectGridRendered), console noise filtering, and Tier-2 trackSessions cleanup. Existing specs are not bulk-migrated; new work imports from here.

CI debugging: Playwright in CI now runs the html reporter alongside list and github, because the github reporter alone never populated playwright-report/. ci.yml and e2e-live.yml upload both runner/playwright-report/ and runner/test-results/ (traces) on failure, with if-no-files-found: warn.

Adds root fflate devDependency for upcoming specs that assert on downloaded workspace zip contents.

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[danielzytohoc]

Collapsed into #184 (Dan's call: spec-only PRs reviewed as one unit — #184 now bases on master and carries this PR's commits; nothing was dropped). The branch stays for history.