Add support for contingency messaging

[CrisBarreiro]

Task/Issue URL: https://app.asana.com/1/137249556945/project/72649045549333/task/1214489465023873?focus=true
Tech Design URL (if applicable):

Description

Add support for contingency messaging:

• Bottom sheet when visiting YouTube
• Message in settings with hidden toggle

Steps to test this PR

Feature 1

• Fresh install the app
• Go to YouTube ad blocking settings and enable the feature
• Go to Feature Flag Inventory and enable adBlockingExtensionFeature -> enableContingencyMode
• Go to YouTube ad blocking settings and check nothing changed
• Open a YouTube video (you might see ads), check no contingency messaging is shown
• Go to Feature Flag Inventory and enable adBlockingExtensionFeature -> adBlockingUXImprovements
• Go to YouTube ad blocking settings and check the toggle disappeared and there's a contingency message
• Go back and reload the video
• Check a contingency message is shown, Dismiss it
• Navigate to another video
• Check contingency message not shown again
• Go to Feature Flag Inventory and disable and immediately enable adBlockingExtensionFeature -> enableContingencyMode
• Go back and reload the video
• Check contingency messaging is shown again

UI changes

Before	After

---

Note

Medium Risk
User-visible browser and settings behavior tied to remote feature flags, including when script injection is skipped in contingency mode elsewhere in the module; well-tested but worth validating navigation timing and one-shot messaging edge cases.

Overview
Adds contingency messaging when remote flags adBlockingUXImprovements and enableContingencyMode are on, so users are told YouTube ad blocking is temporarily unavailable instead of seeing a broken toggle or silent failure.

On YouTube extension domains, a one-time bottom sheet is shown after page load (deferred until the WebView has window focus); pending shows are cancelled on the next onPageStarted. Whether the message already appeared is persisted in DataStore and resets when contingency mode is turned off.

Ad Blocking settings (v2) switches to a read-only contingency row (warning icon + copy) and hides the block-ads toggle and description; the header status indicator stays off in contingency mode even if blocking would otherwise be enabled.

Contingency copy and v2 settings strings are added across locales (moved out of donottranslate.xml).

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[CrisBarreiro]

• Submit strings for translation #9120
• Add support for contingency messaging #9083 👈 (View in Graphite)
• Extract ad blocking domain matcher #9145 : 2 other dependent PRs (#9097 , #9137 )
• develop

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[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[cmonfortep]

LGTM

Just few recommendations to avoid showing the sheet on race conditions, or on wrong sites.