fix(frontend): prevent Resources page freeze with large libraries

[Ashfaqbs]

Summary

Fixes #403.

With ~100k resource entries, the Resources page fetches the full recursive library (already the intended REST behaviour) and then mounts every entry into the FileManager tree in one go. FileManager has no row virtualization, so building and rendering tens of thousands of DOM nodes blocks the main thread on first paint — the page freezes and appears empty, matching the report exactly.

This PR adds a small guard rather than a full virtualization rewrite (which would touch the tree-building, keyboard nav, selection, and DnD code paths and deserves its own focused PR/discussion): above LARGE_LIBRARY_PROMPT_THRESHOLD (5000) entries, if there's no active search query, the page shows a "Large resource library — search to narrow it down" prompt instead of mounting FileManager with the full unfiltered set. Once the user types a query, the existing FileManager render path (including its own client-side filtering) is used unchanged.

This directly prevents the freeze-on-load scenario from the issue without touching FileManager internals, the REST/GraphQL contract, or any of the move/copy/search logic that assumes the full resource list is available in memory.

• frontend/src/features/resources/resources-constants.ts: new LARGE_LIBRARY_PROMPT_THRESHOLD constant.
• frontend/src/features/resources/resources-utils.ts: new pure shouldPromptToSearchLargeLibrary helper.
• frontend/src/pages/resources/resources.tsx: use the helper to swap in a prompt state instead of mounting FileManager when the library is too large and unfiltered.

Full virtualization of the tree would still be worth doing as a follow-up for a smoother browsing experience on huge libraries, but this is a self-contained fix for the reported crash.

Test plan

• Added resources-utils.test.ts covering the new helper's threshold/query logic
• pnpm exec tsc --noEmit — clean
• pnpm exec eslint on all changed files — clean
• pnpm exec prettier --check on all changed files — clean
• pnpm exec vitest run — new tests pass; ran the full suite and confirmed the pre-existing failures (review-sandbox.unit.test.ts, markdown-editor-field.test.tsx) are present on an unmodified checkout too, unrelated to this change