Allow custom JavaScript interfaces in the reflowable Web navigator

[aiviapp]

Closes part of #830.

Problem

The new Compose Web navigators give an app no way to reach the WebView they render into, so features built on top of the legacy navigator have no path forward. docs/guides/navigator/epub-feature-parity.md already records it:

| Custom JavaScript interfaces | ✅ | ❌ | Legacy registerJavascriptInterface has no equivalent |

Change

Adds ReflowableWebConfiguration.javascriptInterfaces:

ReflowableWebConfiguration(
    javascriptInterfaces = persistentMapOf(
        "MyBridge" to { href -> MyBridge(href) }
    )
)

It mirrors EpubNavigatorFragment.Configuration.registerJavascriptInterface, including the null-to-skip-this-resource semantics, but is keyed by Url rather than Link since the new navigators do not use Link. The interfaces are registered in ReflowableResource, in the same LaunchedEffect where the navigator already attaches GesturesApi, DocumentStateApi and SelectionListenerApi to the resource web view.

JavascriptInterfaceFactory lives in readium-navigator-web-common so the fixed-layout navigator can adopt the same field later.

Notes

• Configuration only — no behaviour changes for anyone who does not set the new field.
• The parity table is moved from ❌ to ⚠️ rather than ✅, since FixedWebConfiguration still has no equivalent. Happy to extend this PR to the fixed-layout navigator if you would rather land both together.
• Verified locally with ./gradlew :readium:navigators:web:readium-navigator-web-reflowable:compileDebugKotlin and ./gradlew ktlintCheck.

Why this matters to us

We maintain a Flutter wrapper around the toolkit for a reading app whose text-to-speech is generated outside Readium. On the legacy navigator we use registerJavascriptInterface for a bridge that reports image taps and narration sync back from injected JS. The new reflowable navigator is attractive to us for continuous scrolling across resources, which the legacy one cannot do — but without an extension point on the web view, migrating would mean dropping features we ship today. #830 lists the other three gaps we hit; this is the one small enough to send as a patch.

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