test: rebuild e2e suite on semantic selectors with real assertions

[nick-transition]

Summary

The Playwright e2e suite navigated by hardcoded screen coordinates and its only assertion was body.innerHTML.length > 100, so it could not fail on wrong content — and its coordinates were already stale (they clicked a home "Workouts" tab that no longer exists). This PR rebuilds it on Flutter web's semantics tree with real assertions. The suite now runs in ~9s locally (was minutes of fixed sleeps).

Changes

• Semantic selectors: lib/main.dart force-enables the semantics tree behind --dart-define=ENABLE_SEMANTICS=true; specs target widgets by accessible role/name via shared helpers in e2e/helpers.ts instead of clickAt(x, y) + waitForTimeout
• Real read-only assertions: coach views now assert absence of Edit workout / Start Session / Delete Session and presence of the Coach View chip
• Hermetic runs: e2e/global-setup.ts clears the auth+firestore emulators and reseeds before every run
• Config hardening: CI retries, trace on first retry, video retained on failure; README screenshot generation split into a separate Playwright project (npm run e2e:screenshots) so CI runs only the regression suite
• CI: wait for the Firestore emulator too, cache emulator jars, upload Playwright report + traces
• DX: run_e2e.sh kills emulators it started, gains --skip-build; new npm run e2e / e2e:quick / e2e:screenshots; unused integration_test dependency dropped; local run artifacts gitignored/untracked

Bugs found by the new assertions

1. SessionCard had no tap semantics — assistive tech could not open session details at all. Fixed by exposing the card as a semantic button (lib/widgets/session_card.dart).
2. Seeder wrote session entries as an array field while the app reads the entries subcollection, so seeded session details always showed "No exercises recorded". Fixed in scripts/seed_emulator.js.

README screenshots regenerated to match the current 3-tab UI.

Test plan

• npx playwright test --project=e2e — 4/4 pass locally against emulators
• npx playwright test --project=screenshots — passes, screenshots verified non-blank
• flutter analyze clean for changed files; flutter test failures are pre-existing on main (tracked separately)

🤖 Generated with Claude Code