fix: take over foreign Service Worker scope when the served app changes

[drgould]

Summary

• mdp's routing Service Worker registers at scope /, shared by every repo that ever runs mdp run on a given proxy port over time
• a leftover SW from a previous app on that port keeps intercepting fetches (and serving from its own Cache Storage) for whatever app is served here now, producing mismatched-build breakage
• widget.js now detects when the served app (window.__mdpServedBy) has changed since the last visit (per-port fingerprint in localStorage) and takes over: unregisters any root-scoped foreign SW, clears Cache Storage, then does one guarded reload — a legitimate SW belonging to the current app is left alone

Test plan

• go build ./...
• go vet ./...
• go test ./...