feat: set DuckDuckGo as new tab page

[rgarcia]

Summary

• Add NewTabPageLocation Chrome policy to display DuckDuckGo's start page when opening new tabs
• This complements [kernel-834] update default browser to DuckDuckGo #128 which set DuckDuckGo as the default search provider for the address bar

After #128, the address bar searches use DuckDuckGo, but the new tab page was blank. This PR sets the new tab page to https://start.duckduckgo.com/ which shows:

• DuckDuckGo logo
• Search box with "Search privately" placeholder
• Search/Duck.ai tabs
• "Protection. Privacy. Peace of mind." tagline

Test plan

• Tested manually against production by modifying policy.json on a running browser
• Verify new tab page shows DuckDuckGo start page after image rebuild
• Verify chrome://newtab redirects to https://start.duckduckgo.com/

🤖 Generated with Claude Code

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Note

Sets the Chrome enterprise policy for the new tab page to DuckDuckGo and wires it into server defaults.

• Adds NewTabPageLocation constant and field to Policy with default https://start.duckduckgo.com/
• Initializes NewTabPageLocation in default policy returned by readPolicyUnlocked
• Updates managed policy file shared/chromium-policies/managed/policy.json to include NewTabPageLocation

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