embed discovery walkthrough video

## Summary

Discovery now has the same in-app walkthrough entry point as the report and live pages, so users can open a “how to use discovery” modal from the page header. The modal embeds the final Wistia walkthrough video (`jwvxbs8ja4`) in the app’s shared Wistia player.

## Validation

- `npm test -- 'app/(app)/(dashboard)/discovery'`
- `npx eslint 'app/(app)/(dashboard)/discovery/discovery-client.tsx'`
- `npm run typecheck`
- `git diff --check`
- Local `/discovery` request returned the expected unauthenticated redirect to `/features/reports?from=%2Fdiscovery`.

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[](https://github.com/EveryInc/compound-engineering-plugin)

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> [!NOTE]
> **Low Risk**
> UI-only walkthrough plus a small client-side concurrency guard on discovery scan state; no auth or API contract changes.
>
> **Overview**
> Discovery gets a **page header** with a desktop **“how to use discovery”** button that opens a dialog embedding the shared **`WistiaVideoPlayer`** (media id `jwvxbs8ja4`), matching the in-app help pattern used elsewhere.
>
> The main column layout is adjusted so **`DiscoveryHeader`** sits above **`DiscoveryContent`** (sidebar and mobile drawer wiring unchanged).
>
> **`handleRunScan`** no longer bails out with early `return`s on stale request ids; success and error state updates run only when `scanRequestIdRef.current` still matches the in-flight request, avoiding races when scans are reset or superseded.
>
> ^{Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit 78f8f94d394ecebfab8823acf92721e72e4cc31e. Bugbot is set up for automated code reviews on this repo. Configure [here](https://www.cursor.com/dashboard/bugbot).}