fix(web): hide unsupported Codex slash commands in remote mode

[Sun-sunshine06]

Summary

• stop advertising built-in Codex slash commands in remote web mode until app-server can execute them end-to-end
• show a localized toast when users send unsupported built-ins like /status
• keep project and plugin Codex prompts expandable in autocomplete and add regression coverage

Test Plan

• bun test web/src/lib/codexSlashCommands.test.ts
• bun run --cwd web typecheck
• bun run --cwd web build

[github-actions]

Findings

• None.

Summary
Review mode: initial
No issues found in modified lines. Residual risk: UI path for Codex slash-command blocking/toast not covered by tests.

Testing

• Not run (automation)