ui: Stores consolidation refactor (#27238)
* ui: Remove dead code from stores
- persisted() helper was exported but never used
- messageUpdateCallback / registerMessageUpdateCallback were never wired up
- conversationsStore.initialize() alias, single caller moved to init()
* ui: Merge device, theme and viewport into a single deviceStore
All three are reactive browser-environment signals, now exposed as one
class store: deviceStore.isMobile, deviceStore.isIOSDevice / isIOSSafari
/ isWKWebView / isStandalone and deviceStore.systemTheme.isDark. The
systemTheme name disambiguates the OS preference from the user theme
preference in settingsStore. Drops the unused viewport export (only
isMobile was consumed).
* ui: Merge build info into version store
One VersionStore class with build (llama.cpp build number from
build.json) and frontend (PWA version from _app/version.json),
matching the class pattern of the other stores.
* ui: Colocate context gauge popup state with its components
The gauge popup state is local UI state shared only by the
ChatFormContextGauge subtree, so it lives next to its consumers
instead of the app-scope stores barrel.
