feat(web): simplify Slack sign-in with a configured app

[adampeterhiggins]

Slack settings currently instruct users to create a workspace app even when the environment already has an app client ID configured. Users should be able to sign into a supplied app without going through app creation.

Show direct account sign-in guidance when an app is configured, with an optional Change app form that preserves workspace-specific setup. Cancelling an app edit restores the configured ID. Document shared-app configuration through the existing T3CODE_SLACK_CLIENT_ID setting, workspace approval requirements, and Slack distribution/rate-limit constraints. The existing PKCE authorization, credential storage, remote callback, and disconnect paths are reused.

This does not register a shared Slack app or provide a bundled client ID. An operator must register/configure the app separately; Marketplace approval and workspace approval remain external prerequisites where applicable. Mobile can attach Slack context after connecting through web or desktop, as before.

Validation:

• 13 focused Slack auth/OAuth tests passed, including configured-app sign-in, token exchange, disconnect/reconnect, saved-app precedence, and override cancellation.
• Web and server typechecks passed (server reported existing non-failing Effect suggestions).
• Targeted lint, formatting, and git diff checks passed.
• Draft: browser verification and before/after screenshots skipped at the developer's explicit request. Actual Slack authorization against a distributed app remains unverified.

Model: GPT-6.1-Sol. Harness: Codex in T3 Code.