refactor(desktop): ChatStore structured split + IDB persistence + stream cancel

Split monolithic chatStore.ts (908 lines) into 4 focused stores:
- chatStore.ts: facade layer, owns messages[], backward-compatible selectors
- conversationStore.ts: conversation CRUD, agent switching, IndexedDB persistence
- streamStore.ts: streaming orchestration, chat mode, suggestions
- messageStore.ts: token tracking

Key fixes from 3-round deep audit:
- C1: Fix Rust serde camelCase vs TS snake_case mismatch (toolStart/toolEnd/iterationStart)
- C2: Fix IDB async rehydration race with persist.hasHydrated() subscribe
- C3: Add sessionKey to partialize to survive page refresh
- H3: Fix IDB migration retry on failure (don't set migrated=true in catch)
- M3: Fix ToolCallStep deduplication (toolStart creates, toolEnd updates)
- M-NEW-2: Clear sessionKey on cancelStream

Also adds:
- Rust backend stream cancellation via AtomicBool + cancel_stream command
- IndexedDB storage adapter with one-time localStorage migration
- HMR cleanup for cross-store subscriptions

desktop/src-tauri/src/lib.rs

@@ -117,6 +117,7 @@ pub fn run() {
         .manage(kernel_state)
         .manage(scheduler_state)
         .manage(kernel_commands::SessionStreamGuard::default())
+        .manage(kernel_commands::StreamCancelFlags::default())
         .manage(pipeline_state)
         .manage(classroom_state)
         .manage(classroom_chat_state)
@@ -136,6 +137,7 @@ pub fn run() {
             kernel_commands::agent::agent_import,
             kernel_commands::chat::agent_chat,
             kernel_commands::chat::agent_chat_stream,
+            kernel_commands::chat::cancel_stream,
             // Skills commands (dynamic discovery)
             kernel_commands::skill::skill_list,
             kernel_commands::skill::skill_refresh,