poc: proxy embed links when possible

[Soxasora]

Description

At the moment it only supports Twitter->xcancel and this PR serves as a way to visualize #2664.

ProxyLink detects a supported embed URL and applies the related proxy service (if available), e.g. twitter is replaced with xcancel.

The places where the alternative link is shown:

• Item title with URL
• Embeds

ProxyLink will only show up if the user has enabled proxyEmbeds in the settings: show proxy links for 3rd party embeds

Screenshots

Item title:

Embed:

Settings:

Additional Context

• We can't embed xcancel because of their frame-ancestors: none CSP. So the next best thing is to show a small link that takes the user to a supported proxy.

• I'm not entirely sure that this is something we'd want to comfortably support, considering that proxies can go down. On the other hand, it's a really straightforward solution that doesn't require a bot.

• Since the only supported proxy is for Twitter, ProxyLink is used only on that specific embed. The current Embed structure forces us to repeat ProxyLink on every provider, but it can be refactored to avoid repetitions.

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I noticed that the setting show images, video, and 3rd party embeds is not respected: we still show an embed if this is false.
I'll open an issue

Checklist

Are your changes backward compatible? Please answer below:

For example, a change is not backward compatible if you removed a GraphQL field or dropped a database column.
Yes, added proxyEmbeds to User

On a scale of 1-10 how well and how have you QA'd this change and any features it might affect? Please answer below:
8, it's just a link replace but depends on xcancel status.

For frontend changes: Tested on mobile, light and dark mode? Please answer below:
Yes

Did you introduce any new environment variables? If so, call them out explicitly here:
No

Did you use AI for this? If so, how much did it assist you?
No

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Note

Adds user-controlled proxy links for 3rd-party embeds (currently Twitter → xcancel) and wires it through UI, GraphQL, validation, and DB.

• Embeds/UI
  • Add ProxyLink component and show alongside item URLs and Twitter embeds in components/item.js and components/embed.js.
  • Update settings UI to toggle showing proxy links: proxyEmbeds checkbox in pages/settings/index.js.
• GraphQL/API
  • Extend SettingsInput and UserPrivates with proxyEmbeds in api/typeDefs/user.js.
  • Fetch proxyEmbeds in fragments/users.js.
• Validation
  • Add proxyEmbeds to settingsSchema in lib/validate.js.
• Utilities
  • Implement proxyEmbedUrl and TWITTER_URL_REGEXP; use in parseEmbedUrl in lib/url.js.
• DB
  • Add proxyEmbeds boolean to users (Prisma schema + migration).
• Styles
  • Exclude proxy links from visited color rule using [data-proxy-link] in components/item.module.css.

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