Insights: Fix empty posting activity

[kean]

Description

Fixes CMM-101: Stats: Posting Activity Shows No Data.

Note: the root cause is probably on part 3, but I decided to minimize the affected area and fixed the bug directly.

RCA

The Cache Reset Chain:

1. User navigates to Insights screen

- `SiteStatsInsightsTableViewController.viewWillAppear` subscribes to `StatsInsightsStore`
- Store queries data and populates `state.postingActivity`

3. User taps Posting Activity to view details

- Navigation pushes `PostingActivityViewController` onto the stack
- `SiteStatsInsightsTableViewController.viewWillDisappear` is called
- View controller unconditionally unsubscribes from the store

4. Store detects no active queries and resets state

  // StatsInsightsStore.swift:229-236
  func processQueries() {
      guard !activeQueries.isEmpty else {
          // This being empty means a VC just unregistered from observing data.
          // Let's persist what we have a clear the in-memory store.
          saveDataInCache()
          state = InsightStoreState()  // ← All data cleared!
          return
      }
  }

5. Detail screen finds empty store
   • PostingActivityViewController reads yearData from store
   • Store state was just reset, so postingActivity is nil
   • Collection view renders with 0 items

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[wpmobilebot]

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