Prefer Electron shell before Linux opener fallback for external URLs

[rexlManu]

Summary

• Update openExternalUrl to try Electron shell.openExternal first on all platforms.
• On Linux, fall back to the Linux external opener only if Electron shell throws.
• Add Linux-focused tests for success path, fallback behavior, and full failure behavior.
• Keep non-Linux behavior unchanged (return false when Electron shell fails).

Testing

• Added unit tests in apps/desktop/src/openExternal.test.ts covering:
  • Linux prefers Electron shell when it succeeds.
  • Linux falls back to Linux opener when Electron shell fails.
  • Linux returns false when both openers fail.
• Existing test coverage still includes non-Linux fallback behavior.
• bun lint: Not run
• bun typecheck: Not run