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

codersdk/chats.go

@@ -2411,10 +2411,10 @@ func (c *ExperimentalClient) GetChatsByWorkspace(ctx context.Context, workspaceI
 
 // PRInsightsResponse is the response from the PR insights endpoint.
 type PRInsightsResponse struct {
-	Summary    PRInsightsSummary           `json:"summary"`
-	TimeSeries []PRInsightsTimeSeriesEntry `json:"time_series"`
-	ByModel    []PRInsightsModelBreakdown  `json:"by_model"`
-	RecentPRs  []PRInsightsPullRequest     `json:"recent_prs"`
+	Summary      PRInsightsSummary           `json:"summary"`
+	TimeSeries   []PRInsightsTimeSeriesEntry `json:"time_series"`
+	ByModel      []PRInsightsModelBreakdown  `json:"by_model"`
+	PullRequests []PRInsightsPullRequest     `json:"recent_prs"`
 }
 
 // PRInsightsSummary contains aggregate PR metrics for a time period,