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/database/queries/chatinsights.sql

@@ -173,11 +173,12 @@ JOIN pr_costs pc ON pc.pr_key = d.pr_key
 GROUP BY d.model_config_id, d.display_name, d.model, d.provider
 ORDER BY total_prs DESC;
 
--- name: GetPRInsightsRecentPRs :many
--- Returns individual PR rows with cost for the recent PRs table.
+-- name: GetPRInsightsPullRequests :many
+-- Returns all individual PR rows with cost for the selected time range.
 -- Uses two CTEs: pr_costs sums cost for the PR-linked chat and its
 -- direct children (that lack their own PR), and deduped picks one row
--- per PR for metadata.
+-- per PR for metadata. A safety-cap LIMIT guards against unexpectedly
+-- large result sets from direct API callers.
 WITH pr_costs AS (
     SELECT
         prc.pr_key,
@@ -264,4 +265,4 @@ SELECT * FROM (
     JOIN pr_costs pc ON pc.pr_key = d.pr_key
 ) sub
 ORDER BY sub.created_at DESC
-LIMIT @limit_val::int;
+LIMIT 500;