Added playwright regression test and fixed more bugs - #7

Added a Playwright regression test — and it immediately paid off by catching two more instances of the exact bug class, not just the one I'd already fixed.

The new test (frontend/e2e/smoke.spec.ts, "Kamada-Kawai and Stress layouts run in the background worker on a big network"): posts an 80-node network via /api/external (deterministically over the >60-node worker threshold), runs "kk" then "stress" from the UI, and asserts both
that node positions actually changed and that #layoutStatus is empty (no error). It runs against the real production build, exactly like the rest of this suite (playwright.config.ts's webServer does npm run build && server.py), which is what makes it able to catch
minification-only bugs at all.

What it found: my earlier fix only addressed the dispatcher calling functions by hardcoded name. But getFrWorker()'s pasted function bodies (workStep, frLayout, distanceLayout) also freely reference:

• the shared S state object's S.workRange (for progress reporting) — nonexistent inside the isolated worker, threw " is not defined" ("d is not defined", "_S is not defined", etc., depending on the build)
• the module constants FR_SEED and DIST_LAYOUT_LIMIT, referenced in the worker's own hardcoded-name preamble the same fragile way as before

Both are the identical underlying anti-pattern (a function's .toString()'d body, pasted into an isolated worker, referencing something outside its own scope by a name the minifier is free to rename) — my first fix only handled it for functions (via .name); this pass fixes it for
data:

• workRange/workSink/workLast now live on globalThis.__normaWorkX — globalThis and plain string property names are never renamed, so main thread and worker each get their own correctly-connected copy
• FR_SEED/DIST_LAYOUT_LIMIT are now inlined as literals (123, 2500) at their two pasted-into-worker use sites, with a comment explaining why, while the real exported constants remain the source of truth for every other (main-thread-only) caller