Chat model menu polished and Trace Ingestion moved to Observability

The Chat header now uses a dark, searchable model menu for all 9Router models, and Trace Ingestion has moved out of the Chat area into the existing Observability activity-bar page.

What changed

• index.html replaced the native #brainModelSelect with an app-owned model picker: #brainModelTrigger, #brainModelMenu, #brainModelSearch, and #brainModelList. It also added the data-view="observability" right-panel view and the new #observabilitySlot mount target.
• src/brain-ui.ts now renders the model list as a custom listbox while keeping the existing chatModels, selectedChatModel, and exepert.brainChat.model.v1 persistence flow. The menu supports provider grouping, search, variant badges, Escape, ArrowUp/ArrowDown, Enter, outside-click dismissal, and disabled state while streaming.
• src/ui/observability.ts now prefers #observabilitySlot, falls back to the old #obsSlot only for older shells, and uses exepert.obsCollapsed.v2 so the activity-page panel defaults to expanded.
• css/app.css adds the dark model popover, selected/active row states, responsive badge behavior, Observability workbench view rules, and the wider Trace Ingestion page layout.

User impact

• Opening the Chat model picker no longer shows a white operating-system dropdown in the dark UI.
• Codex and Claude models from 9Router are searchable and grouped by provider.
• Chat keeps its full vertical space because Trace Ingestion is no longer mounted under the chat tile.
• The Observability activity icon now opens a focused Trace Ingestion page with the existing import, sample, trace viewer, live, project, Supabase status, and summary controls.

Verification

Checked locally with:

pnpm typecheck
pnpm build

The browser smoke test confirmed that the menu listed 25 models, filtering worked for Claude models, keyboard selection worked, a chat request used the selected cx/gpt-5.4 model, and #obs-panel mounted inside #observabilitySlot rather than the Chat area.