feat(flow-client): switch the live client bootstrap from GWT to TypeScript

[totally-not-ai]

Summary

Flips the client over to TypeScript. FlowClient.init() (imported and called by Flow.ts) is generated by scripts/client.js; this changes the generator to emit the TypeScript bootstrap (onModuleLoad from Bootstrapper) instead of wrapping the compiled GWT engine blob. The TypeScript engine is now the live client.

The full mocha suite (87 files, 497 tests) — including FlowTests, which exercises the bootstrap — passes against the TypeScript client. The GWT sources and build wiring are removed in the next PR.

⚠️ This is the pivotal PR of the series; its real end-to-end validation is the IT suite (client router, embedding, resync, push). Rollback = revert this one commit.

Stacked on #24952 (base flow-client-ts-engine) — merge after it. Non-draft; please review before merging.

[totally-not-ai]

Review — GWT→TS bootstrap cutover

The client.js build script no longer reads the compiled GWT client-*.cache.js bundle and wraps it; it now generates a small FlowClient.js that imports and calls the TypeScript bootstrap. Verified the full wiring:

• onModuleLoad is exported from src/main/frontend/internal/Bootstrapper.ts (line 231), so the generated import { onModuleLoad } from './internal/Bootstrapper' resolves — the relative path is correct (FlowClient.js is written to src/main/frontend/, Bootstrapper.ts is under internal/).
• Flow.ts dynamically imports ./FlowClient (line 351) and calls clientMod.init() (line 409) behind the isFlowClientLoaded() guard — the same entry point the old GWT bundle exposed.
• The generated init() runs onModuleLoad(), which registers the widgetset start callback so the server bootstrap can start the application and assemble the TS engine (ApplicationConnection) — behaviorally equivalent to the former GWT onModuleLoad.

Clean, minimal, and correct. LGTM. The new comment accurately documents the entry-point flow.