add cross-platform WebView control backed by webview_all

[zaim-tech]

Description

Adds a new cross-platform WebView control backed by webview_all.

It supports:

• Windows through webview_windows
• Linux through webview_cef
• iOS and macOS through native WebView implementations
• Android through the native WebView implementation
• Web through webview_flutter_web

This extends Flet's current WebView support with Windows and Linux support.

The extension is already published independently as flet-webview-all on PyPI. This PR integrates it into Flet as a built-in extension.

Testing status:

• ✅ Windows (desktop) — built and verified locally
• ✅ Web — built and verified locally (flet build web, production build), WebView renders and loads content correctly
• ⚠️ macOS / Linux — not manually tested locally; CI in this PR has not run yet, so platform support here is based on webview_all's existing implementation rather than verified results.

Known limitation: On Web, loading cross-origin content that doesn't send a Cross-Origin-Resource-Policy header can be blocked when served with COEP require-corp (required for WASM builds with threading). This is a constraint of webview_all's Web implementation (iframe-based), not specific to this integration — native platforms are unaffected.

Note on base branch: No active release/v{version} branch was found at the time of opening this PR, so this targets main directly per the current repo state.

Fixes #6530

Test code

import flet as ft
import flet_webview_all as fwa

def main(page: ft.Page):
    page.title = "flet-webview-all"

    page.add(
        fwa.FletWebviewAll(
            url="https://flet.dev",
            expand=True
        )
    )

ft.run(main)

Type of change

• New feature
• Bug fix
• Breaking change
• Documentation update

Checklist

• I signed the CLA.
• I have performed a self-review of my own code.
• My code follows the style guidelines of this project.
• I have commented my code, particularly in hard-to-understand areas.
• My changes generate no new warnings.
• New and existing tests pass locally with my changes.
• I have made corresponding documentation changes, if applicable.
• I have added changelog entries for user-facing changes, if applicable.
• I have updated release guide pages and website/sidebars.yml for breaking changes, removals, and deprecations, if applicable.

Screenshots

Additional details

First-time contributor to Flet core — open to feedback on API naming/conventions for built-in extensions.

Current package exposes the control as flet_webview_all.FletWebviewAll (separate import). Open to renaming/restructuring to ft.WebView(...) or another convention if that better matches Flet's built-in extension naming.

I currently maintain the flet-webview-all GitHub repo and PyPI package independently. If this is accepted as a built-in extension, I'm happy to transfer the repo to the flet-dev org and/or the PyPI package (or add flet-dev as a co-maintainer) — whichever integration pattern (monorepo package vs. standalone flet-dev repo) the team prefers.

Summary by Sourcery

Integrate the cross-platform flet-webview-all extension into Flet as a built-in WebView control.

New Features:

• Add the flet-webview-all extension with a cross-platform FletWebviewAll control for displaying and interacting with web content on Windows, macOS, Linux, iOS, Android, and Web.
• Expose typed WebView events and asynchronous controls for navigation, JavaScript execution, scrolling, caching, cookies, and developer tools.

Enhancements:

• Integrate the WebView extension into Flet's client, Python workspace, build examples, and extension registration.

Build:

• Include flet-webview-all in build dependencies and package publishing validation.

CI:

• Add the extension to CI dependency and publication checks.

Documentation:

• Add user-facing WebViewAll documentation, API reference pages, and an example application.
• Record the new extension in the changelogs.

Tests:

• Add an integration test covering WebViewAll HTML rendering.

[sourcery-ai]

Hey - I've reviewed your changes and they look great!

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