perf(activity): page, summarize and reconcile activity history in Main by index

Schema 57 rebuilds activity_history_records with order_seq (newer = larger;
a move to the front assigns a new maximum) and stored generated columns
(record_id, occurrence, status, status_group, kind, request_id,
conversation_id, project_id, created_at) with indexes. The order is migrated
from record_order_json exactly; legacy duplicates keep their occurrence and
only the first is shown. record_order_json is no longer written.

ActivityHistoryRepository owns the SQL: page({before, limit <= 500, filter,
projectId, conversationId}) newest first by index, summary() with the counts
and conversation titles/status the Activity page derived from the full list,
reconcile() joined with tasks with a renderer-localized detail line, update()
applied directly in SQL without reading the history, replace() for the
legacy migration, clear(). New activity-history:page/summary/reconcile IPC
with zod validation; page and summary run on the readonly worker.
perf:main measures 24,000 records: startup load 106 ms on Main (14 MB) ->
first page + summary 13.5 ms on the worker (0.6 ms loop block, 114 KB).

src/main/ipc.ts

@@ -130,6 +130,9 @@ import {
 import { registerMagicNotesAnalysisIpcHandlers, registerMagicTodosAnalysisIpcHandlers } from './magic-notes/magic-notes-analysis-ipc'
 import { registerMagicNotesIpcHandlers, registerMagicTodosIpcHandlers } from './magic-notes/magic-notes-ipc'
 import {
+  activityHistoryPageRequestSchema,
+  activityHistoryReconcileRequestSchema,
+  activityHistorySummaryRequestSchema,
   assistantIdSchema,
   executionStatsInputSchema,
   conversationBranchInputSchema,
@@ -6873,6 +6876,23 @@ export function registerIpcHandlers(
     assertTrustedSender(event, window)
     assistantDatabase.clearActivityHistory()
   })
+  registerHandler(ipcChannels.activityHistoryPage, (event, input: unknown) => {
+    assertTrustedSender(event, window)
+    return assistantDatabase.getActivityHistoryPageAsync(activityHistoryPageRequestSchema.parse(input))
+  })
+  registerHandler(ipcChannels.activityHistorySummary, (event, input: unknown) => {
+    assertTrustedSender(event, window)
+    return assistantDatabase.getActivityHistorySummaryAsync(activityHistorySummaryRequestSchema.parse(input))
+  })
+  registerHandler(ipcChannels.activityHistoryReconcile, (event, input: unknown) => {
+    assertTrustedSender(event, window)
+    const request = activityHistoryReconcileRequestSchema.parse(input)
+    // Requests still running in Main are active too, whichever window started them.
+    return assistantDatabase.reconcileActivityHistory({
+      ...request,
+      activeRequestIds: [...new Set([...request.activeRequestIds, ...activeRequests.keys()])]
+    })
+  })
 
   registerHandler(ipcChannels.tokenUsageSummary, (event) => {
     assertTrustedSender(event, window)