perf(activity): keep only loaded activity pages in the renderer, driven by Main's summary

The renderer no longer holds the whole history. Startup sends queued
changes, lets Main reconcile leftover pending/running records against the
stored tasks (same localized interruptedOnRestart line), then loads the
first page (200). The Activity page loads a full visible batch (500, as
before paging); "load more" shows the next batch and loads the next page
behind it; a filter switch refetches that filter's first page; leaving the
page trims to one batch. Counts, project/conversation groups, titles,
conversation status and the clear count come from activityHistory.summary
(refreshed on entry, by the page timer, the refresh button and after load
more / filter changes).

Records that are not loaded are still updated incrementally: the update
contract gains content-matched changes (upsert-call, update-request,
resolve-approval, settle-request, remove-request, remove-call) that Main
applies in SQL to shown rows; the store applies the same semantics to its
pages through the shared reference. update returns Main's stored call
records so a call whose earlier record was not loaded keeps one id. Task
refresh reconciles via Main SQL. Legacy localStorage records migrate as
incremental changes; the renderer no longer calls replace.

Tests: randomized sequences of every action, page loads, trims, flushes and
injected failures against the former full-list code on a real database;
first page <= 200 after startup; flush payload depends only on changed
records (including unloaded ones); ActivityPanel output with pages + summary
equals the full-list rendering for randomized datasets with duplicates;
restart interrupts leftover records with the same detail; clear empties the
database, pages and summary; SQL change semantics equal the reference.

src/main/assistant/assistant-database.ts

@@ -41,6 +41,7 @@ import type {
   ActivityHistoryPage,
   ActivityHistorySnapshot,
   ActivityHistorySummary,
+  ActivityHistoryUpdateResult,
   ConversationQueueItem,
   ConversationAnsweredQuestion,
   ConversationBranchInput,
@@ -5310,8 +5311,8 @@ export class AssistantDatabase {
   }
 
   /** Applies incremental changes in one transaction (PERF-15). */
-  updateActivityHistory(input: unknown): void {
-    this.activityHistory().update(input)
+  updateActivityHistory(input: unknown): ActivityHistoryUpdateResult {
+    return this.activityHistory().update(input)
   }
 
   /** Deletes every activity record in one transaction. */