feat: stack insights tables vertically and paginate Pull requests table (#24198)

The "By model" and "Pull requests" tables on the PR Insights page
(`/agents/settings/insights`) were side-by-side at `lg` breakpoints, and
the Pull requests table was hard-capped at 20 rows by the backend.

- Replaced `lg:grid-cols-2` with a single-column stacked layout so both
tables span the full content width.
- Removed the `LIMIT 20` from the `GetPRInsightsRecentPRs` SQL query so
all PRs in the selected time range are returned.
- Can add this back if we need it. If we do, we should add a little
subheader above this table to indicate that we're not showing all PRs
within the selected timeframe.
- Added client-side pagination to the Pull requests table using
`PaginationWidgetBase` (page size 10), matching the existing pattern in
`ChatCostSummaryView`.
- Renamed the section heading from "Recent" to "Pull requests" since it
now shows the full set for the time range.
<img width="1481" height="1817" alt="image"
src="https://github.com/user-attachments/assets/0066c42f-4d7b-4cee-b64b-6680848edc68"
/>


> 🤖 PR generated with Coder Agents

coderd/exp_chats.go

@@ -5626,7 +5626,7 @@ func (api *API) prInsights(rw http.ResponseWriter, r *http.Request) {
 		previousSummary database.GetPRInsightsSummaryRow
 		timeSeries      []database.GetPRInsightsTimeSeriesRow
 		byModel         []database.GetPRInsightsPerModelRow
-		recentPRs       []database.GetPRInsightsRecentPRsRow
+		recentPRs       []database.GetPRInsightsPullRequestsRow
 	)
 
 	eg, egCtx := errgroup.WithContext(ctx)
@@ -5674,11 +5674,10 @@ func (api *API) prInsights(rw http.ResponseWriter, r *http.Request) {
 
 	eg.Go(func() error {
 		var err error
-		recentPRs, err = api.Database.GetPRInsightsRecentPRs(egCtx, database.GetPRInsightsRecentPRsParams{
+		recentPRs, err = api.Database.GetPRInsightsPullRequests(egCtx, database.GetPRInsightsPullRequestsParams{
 			StartDate: startDate,
 			EndDate:   endDate,
 			OwnerID:   ownerID,
-			LimitVal:  20,
 		})
 		return err
 	})
@@ -5788,10 +5787,10 @@ func (api *API) prInsights(rw http.ResponseWriter, r *http.Request) {
 	}
 
 	httpapi.Write(ctx, rw, http.StatusOK, codersdk.PRInsightsResponse{
-		Summary:    summary,
-		TimeSeries: tsEntries,
-		ByModel:    modelEntries,
-		RecentPRs:  prEntries,
+		Summary:      summary,
+		TimeSeries:   tsEntries,
+		ByModel:      modelEntries,
+		PullRequests: prEntries,
 	})
 }