#8546: Fix/config UI improvements

<!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR improves the Config UI by adding a richer “Models” settings experience (provider cards, tooltips, and gateway catalog integration) and enhancing agent model selection to pull from `models.list` instead of only config-defined options. It also updates config snapshot handling so the UI can display a default config when no file exists and a more informative placeholder when reads fail. Key touchpoints: - Backend config IO now returns a default `raw` payload (and an error placeholder) so the UI can render something even when the config file is missing/unreadable. - UI state/controllers add `modelCatalog` + loading state, fetching the model list from the gateway and wiring it into Agents and Config views. - Config form gains tooltip support and a dedicated models settings view that uses the gateway catalog and detected provider auth hints. <h3>Confidence Score: 3/5</h3> - This PR is mostly safe to merge, but there are a couple correctness/UX edge cases around model ID formatting and catalog-unavailable behavior. - Most changes are additive UI and state plumbing. The main correctness risk is how catalog models are converted into config IDs (can double-prefix provider, breaking selection). There are also UX edge cases when the catalog isn’t available (fallback selection becomes unusable) and the new default raw config uses JSON5-style comments which may be saved as-is. - ui/src/ui/views/agents.ts; src/config/io.ts <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} <!-- /greptile_comment -->