fix(chat): share one WebView asset loader instead of one per mount

[hydall]

Follow-up to #390, which cut the per-rebuild allocation but left one leaked AssetsPathHandler per chat surface mount.

chatWebViewAssetLoader() built a fresh WebViewAssetLoader on every call. Each AssetsPathHandler registers a MethodChannel under a unique name in its _init and installs a method call handler on the binary messenger; nothing disposes it, and the binding's handler map holds it for the process lifetime.

On Android the chat WebView is kept alive and reused. FlutterWebViewFactory.create returns the cached FlutterWebView for a known keepAliveId and only constructs a new one — the only place initialSettings is parsed — when there is no cached instance. Every mount after the first therefore handed the native side settings it never reads, while the handler they carried stayed registered on the Dart side.

Disposing per mount would be the wrong shape: whichever mount created the native WebView owns the live handler, and in practice that is the preloader, not the surface. Sharing one loader sidesteps the ownership question entirely — the native side only ever uses one, and handle() is stateless.

Changes

• chat_webview_settings.dart: chatWebViewAssetLoader() returns a lazily created shared WebViewAssetLoader instead of a new one per call.

Verification

Profile build on a Pixel 8 Pro emulator (Android 16, x86_64) with a seeded 600-message chat, driven over adb; heap sampled through the Dart VM service with getAllocationProfile(gc: true).

• AssetsPathHandler across 30 chat open/close cycles: 1 → 1 → 1 → 1 (+0, +0, +0). MethodChannel flat at 23. For comparison: +190 per 10 cycles before fix(chat): build the WebView settings once per surface, not per rebuild #390, +10 after it.
• Dart heap over the same 30 cycles: 88,342,144 → 88,342,272 → 88,341,920 bytes — flat.
• Cold start into the chat renders correctly: styles, italics, quote colouring, avatars, Context card. No asset-loading or console errors in logcat, so the shared handler serves appassets.androidplatform.net as before.
• flutter analyze — 9 issues, all info/warning, identical to the nightly baseline, 0 errors.
• flutter test — 3812 pass, including chat_webview_keep_alive_test.dart and chat_webview_security_test.dart, which cover these settings helpers. The same 4 failures as on unpatched nightly remain and are local-only (the contract test needs CI's easy_localization:generate step; the clipboard cases pass in CI on ubuntu).

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