fix: normalize BGL bridge submit endpoint

[tolga-tom-nook]

Summary

Normalizes the BGL bridge submit endpoint before appending /submit/bgl.

The default bridge endpoint includes a trailing slash:

this.bridgeEndpoint = config.bridgeEndpoint || 'https://bglswap.com/app/'

But _submitToBridge appended another slash:

`${this.bridgeEndpoint}/submit/bgl`

That produces https://bglswap.com/app//submit/bgl for the default/configured endpoint. This PR trims trailing slashes first so both https://bglswap.com/app/ and https://bglswap.com/app resolve to the canonical https://bglswap.com/app/submit/bgl URL.

Validation

Focused regression test:

npx jest src/__tests__/BGLBridgeEndpoint.test.ts --runInBand --coverage=false

Result:

PASS src/__tests__/BGLBridgeEndpoint.test.ts
Test Suites: 1 passed, 1 total
Tests: 2 passed, 2 total

Targeted lint:

npx eslint src/bridge/BGL.ts src/__tests__/BGLBridgeEndpoint.test.ts --ext .ts

Result: passed.

Diff hygiene:

git diff --check

Result: clean.

Note: npm run build is currently blocked by the repo's dependency/toolchain mismatch after fresh install (@types/node requires disposable lib symbols not available to the pinned TypeScript 4.9 toolchain). The focused Jest regression and targeted ESLint pass for this change.

Bounty context: Bitgesell PR bounty/improvement program.