Move the hardcoded feed endpoint placeholder to the i18n bundle

[Schro-code]

Why

The custom source endpoint placeholder was hardcoded as "https://your-service/utopia-feed" — the only hardcoded English string in the source modal. All sibling fields (endpointField, endpointCopied, copyEndpoint) already use S.library.* keys.

What changes

• web/src/i18n/en.ts — adds feedPlaceholder: "https://your-service/utopia-feed"
• web/src/i18n/zh.ts — adds feedPlaceholder: "https://your-service/utopia-feed" (same value, it is a URL)
• web/src/pages/Library.tsx — replaces the hardcoded string with S.library.feedPlaceholder

How it was checked

• pnpm build and pnpm test pass locally and in CI
• Placeholder text is visually identical (same string, now from i18n)

Before review

• Every commit is signed off (git commit -s)
• For changes under web/: pnpm build and pnpm test pass
• UI strings are in both web/src/i18n/en.ts and zh.ts

[WaylandYang]

Thank you, and welcome. A small, clean change: the last literal string in that modal now comes from the bundle like its neighbours. CI is green.

Merging. Thanks again.