refactor: replace React Query message cache with windowed message store

Introduces a new message-window-store module to manage message state with
automatic windowing of visible and pending messages. This replaces manual
React Query cache operations with a centralized, observable state system.
close #39

Key changes:
- New MessageWindowState tracks visible/pending messages with size limits
- Automatic trimming of message windows (400 visible, 200 pending messages)
- Pending message buffering when user scrolls away from bottom
- Centralized status updates for optimistic messages
- Thread component simplified with forwarded scroll and pending callbacks
- Removes message count tracking from components

This improves performance for chats with many messages and provides a
cleaner separation of concerns between UI and state management.

web/src/hooks/mutations/useSessionActions.ts

@@ -3,6 +3,7 @@ import { isPermissionModeAllowedForFlavor } from '@hapi/protocol'
 import type { ApiClient } from '@/api/client'
 import type { ModelMode, PermissionMode } from '@/types/api'
 import { queryKeys } from '@/lib/query-keys'
+import { clearMessageWindow } from '@/lib/message-window-store'
 
 export function useSessionActions(
     api: ApiClient | null,
@@ -100,7 +101,7 @@ export function useSessionActions(
         onSuccess: async () => {
             if (!sessionId) return
             queryClient.removeQueries({ queryKey: queryKeys.session(sessionId) })
-            queryClient.removeQueries({ queryKey: queryKeys.messages(sessionId) })
+            clearMessageWindow(sessionId)
             await queryClient.invalidateQueries({ queryKey: queryKeys.sessions })
         },
     })