Add the Bublik AI assistant interface

[okt-limonikas]

Summary

Add an Assistant section with persistent chat threads, model and reasoning
selection, rich streamed responses, cancellation, background-run recovery,
context usage, and generated-file downloads.

Related

• Add the configurable Bublik AI assistant backend bublik#348
• Deploy the Bublik AI assistant bublik-docker#79
• docs(ai): document chat configuration and deployment bublik-release#58

Changes

• Add /chat and /chat/:threadId routes and Assistant sidebar navigation.
• Add model/provider selection with stale-selection recovery.
• Render Markdown, highlighted code, reasoning, tool calls, and structured output.
• Add user thread creation, rename, archive, restore, deletion, and Markdown export.
• Continue displaying server-side runs after navigation or reload.
• Support cancellation and prevent duplicate submission while a run is active.
• Display compaction notifications and model context usage.
• Render and download files produced by the assistant.
• Restrict creation of the global ai configuration to No Project (Default).
• Add unit tests for API schemas, context usage, reasoning grouping, tool state,
  Markdown export, and AI configuration behavior.

Backend Contract

This interface requires the AI chat backend endpoints for models, threads,
streaming generation, cancellation, and generated-file downloads. It also
expects active-run and context-usage fields in thread responses.