[BUG]: Prevent false-positive share success events in Discord fallback

### Bug Description

## Description

The current Discord fallback share flow emits `social_share_success` and triggers the `onShare` callback immediately after calling `copyLink()` and `window.open()`.

If clipboard access fails or the popup is blocked by the browser, the library can still report a successful share even though the fallback action did not complete successfully.

## Expected Behavior

Only emit `social_share_success` and invoke `onShare` after confirming that the fallback action has completed successfully.

## Possible Solution

- Return a success/failure result from `copyLink()`.
- Verify that `window.open()` succeeds.
- Emit the success event only when the fallback action completes successfully.

I'd be happy to work on this Issue

### Steps to Reproduce

_No response_

### Logs and Screenshots

_No response_

### Environment Details

_No response_

### Impact

Critical - Application is unusable

### Code of Conduct

- [x] I agree to follow the Code of Conduct
- [x] I have joined the [Discord server](https://discord.gg/hjUhu33uAn) and will post updates there
- [x] I have searched existing issues to avoid duplicates

Start by locating the TypeScript Discord fallback and its copyLink() and window.open() call sites. Trace how their outcomes reach social_share_success and onShare, then validate both success and failure paths with the repository’s existing test setup; done means neither callback nor event fires when clipboard access or the popup fails.

索引モデルが issue の本文から書いたものです。