fix: resolve Homebrew gt path and prevent activity-feed ENOENT crash

[web3dev1337]

Summary

Fixes issue #25 where GUI crashes on startup/websocket connect when gt is not on PATH (common with Homebrew installs at /opt/homebrew/bin/gt).

Root cause:

• Multiple backend paths hardcoded gt/bd command names.
• Activity feed subprocess had no error listener, so spawn gt ENOENT crashed Node.

Changes:

• Added ExecutableResolver to resolve binaries from:
  • explicit env overrides: GT_BIN, BD_BIN
  • PATH
  • fallback paths including /opt/homebrew/bin/*, /usr/local/bin/*, Linuxbrew
• Wired resolved executables through:
  • server.js executeGT/executeBD
  • onboarding setup version checks
  • websocket activity feed spawn path
  • GTGateway and BDGateway via injectable executable
• Hardened activity feed lifecycle:
  • added process error handler
  • restart scheduling deduplication
  • timer cleanup when last websocket client disconnects
• Added docs for new env vars and infrastructure.

Tests

Baseline before changes:

• npm test (passed on origin/master)

After changes:

• npm test -- test/unit/executableResolver.test.js test/integration/cli-executable-resolution.test.js test/unit/gtGateway.test.js test/unit/bdGateway.test.js
• npm test
• npm run test:e2e

All passing.

Issue Link

Closes #25