docs: add Grid and Tree Grid interaction examples

[totally-not-ai]

Summary

This PR adds documentation examples for common Grid and Tree Grid tasks: selecting rows with the arrow keys, selecting a Tree Grid parent together with all its children, handling data loading errors, exporting Grid data as CSV, and showing an uploaded CSV file in a Grid.

What changed

Docs only. Nothing changes in any library.

• Grid selection (components/grid/selection.adoc): new "Selecting Rows with Arrow Keys" section (Flow) with a live GridSelectOnFocus demo. A cell focus listener selects the focused row, and deselection is turned off.
• Tree Grid (components/tree-grid/index.adoc): new "Selecting Children Recursively" section (Flow) with a live TreeGridRecursiveSelection demo. When the user selects or deselects a parent, all of its descendants from the TreeData are updated too. The listener skips changes that did not come from the client, so it does not react to its own updates. The text also explains how to do this with a custom data provider (fetchChildren) and warns that it can be slow.
• Paginated Grid (add-grid/paginated-data.adoc): new "Handling Errors" section. It shows how to catch service errors in the fetch callback, show a notification with a Retry button (which calls refreshAll()), and return an empty stream.
• Downloads (handle-downloads.adoc): new "Exporting Grid Data" section. It exports the selected rows, or all rows when nothing is selected, as CSV through an Anchor download handler, and escapes values correctly.
• Uploads (handle-uploads.adoc): new "Showing Uploaded CSV Data in a Grid" section. It reads an uploaded CSV file into a Grid<String[]> and makes one column per header value.
• New frontend import file grid-select-on-focus.ts for the Java-only demo.

The Tree Grid filter expansion example has moved to #6197.

API Changes

2 new demo view classes (docs examples only, not library API).

com.vaadin.demo.component.grid.GridSelectOnFocus

// Added
@Route("grid-select-on-focus") public class GridSelectOnFocus extends Div
public GridSelectOnFocus()
public static class Exporter extends DemoExporter<GridSelectOnFocus>

com.vaadin.demo.component.treegrid.TreeGridRecursiveSelection

// Added
@Route("tree-grid-recursive-selection") public class TreeGridRecursiveSelection extends Div
public TreeGridRecursiveSelection()
public static class Exporter extends DemoExporter<TreeGridRecursiveSelection>

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Preview Deployment

This PR has been deployed for preview.

URL: https://docs-preview-pr-6185.fly.dev

Changed pages

Added content is highlighted in green; removed content is marked in red on each page.

• building-apps/forms-data/add-grid/paginated-data — modified, +39/-0 lines
• building-apps/forms-data/handle-downloads — modified, +43/-0 lines
• building-apps/forms-data/handle-uploads — modified, +34/-0 lines
• components/grid/selection — modified, +19/-0 lines
• components/tree-grid — modified, +20/-0 lines

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