fix: #4356 support custom protocol links - #4416

Hi — I came at this issue from the crash side (#4473) and traced the renderer paths while testing, so one heads-up in case it's useful: I believe the new isExternalProtocolUrl branch may be unreachable from the actual UI flows. The popover's jump button sends only { href } (editor.vue jumpClick), and for a custom-protocol link muya sanitizes the href away, so the payload arrives as { href: null } with no text and the handler's early return drops it. The Cmd/Ctrl+click path is guarded inside muya (linkMouseEvents.ts returns early when linkInfo.href is null), so it never emits at all. The e2e tests pass because they send the IPC payload directly with text populated, which the UI never does for these links. Making this work end-to-end probably also needs the renderer to forward text (or muya to expose the unsanitized href). Happy to be proven wrong!

@Phdigital33 still Draft status?

Thanks @Jocs and @FurkaanBoraa. Yes, this should remain Draft for now.

@FurkaanBoraa, I think your read is correct. The current test exercises the main-process IPC path directly, but it does not prove the real editor/popover flow can reach that branch. For custom protocols, Muya appears to sanitize the rendered href to null, and the current jumpClick path only forwards { href }, so the proposed isExternalProtocolUrl handling may not be reachable from normal UI interaction.

I'll rework this before marking it ready for review. My plan is to make the actual renderer/Muya path preserve or forward the original link target in a safe way, then replace/augment the direct IPC test with an end-to-end test that starts from a real markdown link, opens the popover, clicks jump, and verifies the expected external-open behavior.

I'll also revisit the security boundary around opening custom protocols. If maintainers prefer landing the narrower crash fix first via #4473 and treating custom-protocol support as a separate feature with confirmation/preference behavior, I'm happy to adapt this PR accordingly.