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/shared/contracts.ts

@@ -44,7 +44,12 @@ import {
   type AssistantTask,
   type ExecutionStats,
   type ExecutionStatsInput,
+  type ActivityHistoryPage,
+  type ActivityHistoryPageRequest,
+  type ActivityHistoryReconcileRequest,
   type ActivityHistorySnapshot,
+  type ActivityHistorySummary,
+  type ActivityHistorySummaryRequest,
   type ActivityHistoryUpdate,
   type ActivityRecord,
   type TokenUsageSummary,
@@ -2167,6 +2172,15 @@ export type DesktopApi = {
     update: (update: ActivityHistoryUpdate) => Promise<void>
     /** Deletes the whole history in one transaction. */
     clear: () => Promise<void>
+    /** One page of shown records, newest first (see activityHistoryPageRequestSchema). */
+    page: (request: ActivityHistoryPageRequest) => Promise<ActivityHistoryPage>
+    /** Counts and conversation titles/status over the whole history. */
+    summary: (request: ActivityHistorySummaryRequest) => Promise<ActivityHistorySummary>
+    /**
+     * Ends records left pending/running by requests that are no longer
+     * active, against the stored tasks; returns how many changed.
+     */
+    reconcile: (request: ActivityHistoryReconcileRequest) => Promise<number>
   }
   usage: {
     getTokenSummary: () => Promise<TokenUsageSummary>