Add screenshots to the PWA manifest so the install prompt shows the app

Chrome offers a richer install experience — a small preview of the app, rather than a bare name and icon — when the manifest declares screenshots. Toolbox declares none, so both desktop and mobile fall back to the plain prompt.

Chrome says so directly, in DevTools → Application → Manifest on `frappe.tools`:

```
Richer PWA Install UI won't be available on desktop.
Please add at least one screenshot with the form_factor set to wide.
Richer PWA Install UI won't be available on mobile.
Please add at least one screenshot for which form_factor is not set or set to a value other than wide.
```

The live manifest declares `id`, `name`, `short_name`, `description`, `start_url`, `scope`, `display`, `background_color`, `theme_color`, `categories` and `icons`. No `screenshots`.

## Why it is worth having

The install prompt is the one moment a visitor decides whether this is a real application. A name and an icon ask them to take that on trust. A picture of the calculator does not.

It costs two images and a manifest field. Nothing else changes.

## What to add

At least one of each form factor, or Chrome keeps both warnings:

```json
"screenshots": [
{ "src": "/assets/toolbox/pwa/screenshot-wide.png", "sizes": "1280x800", "type": "image/png", "form_factor": "wide", "label": "All tools" },
{ "src": "/assets/toolbox/pwa/screenshot-narrow.png", "sizes": "750x1334", "type": "image/png", "form_factor": "narrow", "label": "Calculator" }
]
```

## Where it goes

The manifest is generated by the build, in `frontend/build/pwaBuildPlugin.js`, so the field belongs there rather than in a checked-in JSON file. The images belong beside the icons in `toolbox/public/pwa/`.

The QA suite can produce them rather than anybody taking them by hand: `frontend/e2e/visual.spec.js` already captures every route at 1280x800 and at 375x812, in both themes, and those images are committed. Two of them are exactly the two form factors Chrome is asking for.

## Also worth doing

`frontend/src/pwa/manifest.test.js` should assert the field exists with both form factors, so this cannot silently regress — and so the next person to read the manifest test learns that Chrome wants it.

## Found by

Opening DevTools on the live site while confirming #271. Neither the QA suite nor CI would have found it: nothing in either reads Chrome's manifest diagnostics, and the manifest is valid without screenshots. Worth remembering as a limit of the automated checks rather than a gap to close — some things only a browser will tell you, and only if you ask it.

Start in frontend/build/pwaBuildPlugin.js and inspect frontend/src/pwa/manifest.test.js to see how the generated manifest is assembled and tested. Review the existing captures in frontend/e2e/visual.spec.js and the assets beside the icons in toolbox/public/pwa/. Done means the manifest includes wide and narrow screenshots, the referenced images exist, and the manifest test covers both form factors.

Written by the indexing model from the issue text.