fix: honor configured WBGL bridge endpoint

[newmattock]

Summary

• Make WBGL honor config.bridgeEndpoint, matching BGL and the wrapper SDK behavior.
• Add a focused regression test proving WBGL swap submission uses a configured bridge endpoint instead of always posting to the production default.

Verification

• pnpm exec jest src/__tests__/WBGLEndpoint.test.ts --runInBand
• pnpm exec tsc --noEmit --skipLibCheck --module commonjs --target es2019 --esModuleInterop --types jest src/bridge/WBGL.ts src/__tests__/WBGLEndpoint.test.ts
• pnpm exec prettier --check src/__tests__/WBGLEndpoint.test.ts
• git diff --check

Notes

• I kept the production default unchanged: https://bglswap.com/app/.
• I did not run the full project test suite because current upstream tests include focused live-bridge tests requiring environment credentials and network/gas-sensitive behavior; this PR uses a mocked bridge response and stubs the token send path.

Bounty context

Submitted for Bitgesell improvement bounty consideration: BitgesellOfficial/bitgesell#39.

[MyTH-zyxeon]

Maintainers: this looks like a good #39 / #81 candidate because WBGL currently appears to ignore config.bridgeEndpoint while the BGL side and wrapper SDK path can be configured. The diff keeps the production default unchanged and adds a focused mocked regression, so it is reviewable without touching live bridge funds.

Acceptance checks I would focus on:

• Confirm trailing-slash behavior is intentional. The test uses https://bridge.example.test/app/ and expects /submit/wbgl; a config without the trailing slash may still produce a malformed endpoint unless callers normalize it elsewhere.
• Confirm config.bridgeEndpoint || default should treat an empty string as default. If empty string should be rejected instead, this constructor is the right boundary to make that explicit.
• The test stubs _sendWbgl and mocks fetch, which is a safe unit-level proof. Before merge, I would still want one non-secret dry-run or integration note showing the endpoint override path in a realistic SDK initialization.
• No production default changed: https://bglswap.com/app/ remains the fallback.

No wallet keys, funding, live bridge calls, gas-sensitive actions, or secrets are involved in this review note.