feat(websearch): add DuckDuckGo as free local web search provider

[Shiina18]

Add DuckDuckGo as a new web search provider option. DuckDuckGo is a free local provider following the same pattern as Baidu and Bing.
#13373

Changes:

• Add LocalDuckDuckGoProvider implementation
• Add DuckDuckGo logo asset
• Register DuckDuckGo in WebSearchProviderFactory
• Add DuckDuckGo configuration in webSearchProviders.ts
• Add migration (v200) to add DuckDuckGo provider for existing users
• Update UI components to display DuckDuckGo logo

What this PR does

Before this PR:

Cherry Studio did not support DuckDuckGo as a web search provider.

After this PR:

Users can now select DuckDuckGo as a web search provider in settings. DuckDuckGo is a free local provider. It is a good option for users who want a free, privacy-oriented search backend alongside existing options like Baidu and Bing.

Fixes #

Why we need it and why it was done in this way

The following tradeoffs were made:

• Added migration (v200) to automatically add the DuckDuckGo provider for existing users, following the same pattern as other local providers.

The following alternatives were considered:

• None

Links to places where the discussion took place: N/A

Breaking changes

None

Special notes for your reviewer

N/A

Checklist

This checklist is not enforcing, but it's a reminder of items that could be relevant to every PR.
Approvers are expected to review this list.

• PR: The PR description is expressive enough and will help future contributors
• Code: Write code that humans can understand and Keep it simple
• Refactor: You have left the code cleaner than you found it (Boy Scout Rule)
• Upgrade: Impact of this change on upgrade flows was considered and addressed if required
• Documentation: A user-guide update was considered and is present (link) or not required. Check this only when the PR introduces or changes a user-facing feature or behavior.
• Self-review: I have reviewed my own code (e.g., via /gh-pr-review, gh pr diff, or GitHub UI) before requesting review from others

Release note

Add DuckDuckGo as a new free web search provider option. No API key required; uses local browser-based search like Baidu and Bing.

[MaoJianwei]

Great