hotfix: probe target gateway URL during TUI startup launch

[Yohjisakamoto]

Pull request type

Choose one primary type. If the change needs more focused questions, use the matching template in
PULL_REQUEST_TEMPLATE/:

• Bug fix
• Feature or behavior change
• Performance or efficiency claim
• Provider compatibility
• Documentation only
• Maintenance or other

Outcome

Start with the result a user or maintainer can observe, and link the issue when
required. Then explain why the change is no larger than it needs to be.

Scope and compatibility

• Changed contracts, state, storage, protocol, CLI, GUI, TUI, or installation behavior:
• Compatibility or migration risk:
• Explicitly out of scope:

Verification

List the exact commands and summarized results. Name any skipped affected OS,
surface, provider/protocol, behavior, or state cells and why. Do not enumerate
unaffected combinations.

command -> result

Type-specific evidence

Complete only what applies:

• Bug: reproduction, root cause, and smallest owning regression layer.
• Feature: current user problem and observable acceptance criteria.
• Performance: claim plus the benchmark fields in the
  contribution guide.
• Provider: protocol fixture/live boundary and external dependency metadata.
• Documentation: owning sources checked and links/rendering verified.

If stable automation was not possible, explain why and give the durable
substitute evidence.

Safety and responsibility

• No credentials, private session data, unsafe provider logs, or generated local state are included.
• Public evidence was sanitized according to the
  contribution guide.
• A human is the primary submitter and accepts responsibility for correctness, licensing, provenance, verification, and the statements in this pull request.
• I have the right to submit all included code, data, prompts, fixtures, and generated material under the repository license.

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