Improve Edge send-only compatibility handling

[insane66613]

Summary

This PR improves Microsoft Edge compatibility handling for the Join Chrome extension while preserving existing Chrome behavior.

It is not intended to make Edge a full Join cloud-push receiver. Edge does not provide the legacy Chrome extension receiver APIs used by Join's current browser receiver path (chrome.gcm / chrome.instanceID). A true Edge receiver would require a different Web Push/FCM registration and delivery path, including backend compatibility.

Instead, this PR makes unsupported receiver capability degrade more cleanly and adds clearer send-only behavior for browsers that cannot register as full cloud-push receivers.

Key changes

• Adds Edge-aware compatibility handling across background/offscreen contexts.
• Adds send-only fallback behavior for browsers that cannot register as full cloud-push receivers.
• Improves cross-context calls and service-worker/offscreen compatibility paths.
• Updates options UI messaging for unsupported cloud-push receiver mode.
• Adds/updates test harness coverage for Edge/send-only conditions.
• Updates patched extension metadata/version label.

Important limitation

This PR does not implement a replacement receiver transport for Edge. Join's existing browser receiver implementation depends on Chrome-specific legacy APIs. The safe scope here is compatibility hardening and send-only fallback, not full Edge receive support.

Validation

Repository has no package.json/CI test runner. Local validation performed:

• Static review of the changed receiver/cross-context paths.
• Syntax checks on the touched JavaScript files where practical.
• Repo AI review of the branch and follow-up receiver-capability hardening.

Manual browser validation is still recommended before merge:

1. Load unpacked in Chrome and confirm existing receiver behavior still works.
2. Load unpacked in Edge and confirm the extension does not crash and clearly falls back to send-only where receiver registration is unavailable.
3. Use the in-extension tests/options page to confirm send/receive status messaging.