PUB-4: Remove poster proxy setting

[rovkinmax]

Summary

• Removed the poster proxy entry point from device settings UI, actions, state, VM handling, and resources.
• Deleted PosterUrlMapper and its Koin binding, and removed poster proxy preference persistence.
• Updated video/player UI mapping and screen-scoped mapper construction to use direct HTTPS-normalized poster URLs.

Compliance Review

• Mandatory Puber compliance review passed for PUB-4.
• No compliance violations were found against AGENTS.md, .kent/project-contract.md, .kent/commands/compliance-review.md, and .todo/refactor-remove-poster-proxy/plan.md.

Verification

• ./tools/agentw :app:compileDevDebugKotlin :app:testDevDebugUnitTest - BUILD SUCCESSFUL in 295ms.
• Repository search for posterProxy|PosterProxy|poster_proxy|settings_poster_proxy|PosterUrlMapper|toProxyUrl|KEY_POSTER_PROXY|poster_proxy_enabled under app/src/main and app/src/test returned no matches during compliance review.

Skipped Checks / Blockers

• Device smoke testing was not run; this refactor only removes the poster proxy setting and mapping path.
• No blockers.