ui: Stores architecture improvements (#26910)
* refactor: Stores barrel imports + SSR gates
* refactor: Drop agenticStore wrapper exports
* refactor: Drop chatStore wrapper exports
* refactor: Drop modelsStore wrapper exports
* refactor: Drop serverStore wrapper exports
* refactor: Drop unused mcpStore wrapper exports
* refactor: Drop mcpResourceStore wrapper exports
* refactor: Drop conversationsStore wrapper exports + move buildConversationTree to utils
* refactor: Drop settingsStore wrapper exports
* refactor: Drop unused toolsStore wrapper exports
* refactor: Fix lint errors from store wrapper removal
* fix: Missing change
* refactor: Cleanup
* refactor: Context Stats store
