Add per-site GutenbergKit opt-in and announcement bottom sheet

[jkmassel]

Summary

• Adds a one-time announcement bottom sheet that surfaces the upcoming GutenbergKit editor on app start, with Try it now / Maybe later and an inline Learn more link.
• Adds a per-site Use new block editor toggle in Site Settings so users can opt in or out at any time.
• Persists per-site opt-in state and a one-shot "announcement shown" flag in AppPrefs.
• Routes opted-in sites into GutenbergKit from EditorLauncher.
• Gated on the remote feature flag (isGutenbergKitRemoteFeatureEnabled) and suppressed for sites where the block editor isn't the default (via SiteUtils.isBlockEditorDefaultForNewPost).

Changes

• GutenbergKitAnnouncementBottomSheetFragment + gutenberg_kit_announcement_bottom_sheet.xml: New bottom sheet. Consumes top window insets so Material's BottomSheetDialog doesn't double-pad under the status bar. Inline "Learn more" ClickableSpan opens a web page via WPWebViewActivity.
• WPMainActivityViewModel: New checkAndShowGutenbergKitAnnouncement(site) — gated on remote flag, one-shot pref, non-null site, and SiteUtils.isBlockEditorDefaultForNewPost(site).
• WPMainActivity: Observes the announcement event and shows the fragment.
• GutenbergKitFeatureChecker: FeatureState now carries isEnabledForSite; getFeatureState / isGutenbergKitEnabled accept an optional SiteModel. @JvmOverloads added for Java interop. New isGutenbergKitRemoteFeatureEnabled() for opt-in surfaces.
• SiteSettingsFragment: New preference wired to AppPrefsWrapper.isGutenbergKitEnabledForSite / setGutenbergKitEnabledForSite. Visibility gated by the remote flag.
• AppPrefs / AppPrefsWrapper: Adds GUTENBERG_KIT_ANNOUNCEMENT_SHOWN and the per-site opt-in key.
• EditorLauncher: Passes the current site into the feature check so the per-site opt-in is honored.

Test plan

• Fresh install with remote flag on: launch app → announcement sheet appears once; tap Try it now → editor opens in GutenbergKit; sheet does not re-appear on next launch.
• Maybe later dismisses without opting in; sheet does not re-appear.
• Site Settings: toggling Use new block editor opts the current site in/out; survives app restart.
• Site with block editor disabled (mobileEditor == "aztec"): announcement sheet does not appear.
• Remote flag off: neither the announcement nor the Site Settings toggle appear.
• Learn more inline link opens the web page.
• Unit tests: WPMainActivityViewModelTest, GutenbergKitFeatureCheckerTest pass.

[dangermattic]

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⚠️	This PR is larger than 300 lines of changes. Please consider splitting it into smaller PRs for easier and faster reviews.

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