fix(webview): render inline HTML on web instead of permanent Loading state

[cursor]

Bug and impact

Trigger: A screen uses EnsembleWebView with the new html property (merged in PR #2291) on web.

Impact: The widget shows Loading... forever and never renders the HTML. Native/mobile works because only native/webviewstate.dart was updated; the web iframe path still required url and treated url == null as unloaded.

Root cause

PR #2291 added html, htmlBaseUrl, and injectedJavaScript to the controller and native InAppWebView (initialData), but the web implementation (web/webviewstate.dart) was not updated. Setting html clears url, so buildWidget always hit the Loading... branch.

Fix

• Load inline HTML on web via iframe srcdoc, using shared helpers for base URL + optional injected script (matching native defaults).
• Update iframe content when html / htmlBaseUrl / injectedJavaScript change.
• Use the resolved base URL for postMessage origin checks when in HTML mode.
• Add unit tests for the shared document/origin helpers.

Validation

• Added modules/ensemble/test/webview_html_content_test.dart.
• Run from modules/ensemble: flutter test test/webview_html_content_test.dart
• Flutter SDK was not available in the automation environment; please verify in CI.

Duplicate check

• Merged PR feat(webview): add HTML content and JavaScript injection support #2291 introduced html support but only for native — this completes the web path for the same feature.
• Open PR security(webview): restrict native JS channel callbacks to configured origin #2285 (security): native JS channel origin validation — different code path/concern.
• Open PR fix(wifi, action): correct callback routing and async apiMap restore #2286: WiFi callbacks + executeAction await — unrelated.
• Open PR fix(cdn): defer lastUpdatedAt commit until manifest fetch succeeds #2284 / fix(cdn): reject manifests missing artifacts before cache wipe #2288: CDN refresh failures — unrelated.
• Open PR fix(firestore): dedupe listenForChanges subscriptions to prevent crash #2257: Firestore listen dedup — unrelated.
• No existing open PR, branch, or issue targets web inline HTML rendering failure.