fix(web): restore empty-query skill discovery

[codeblackhole1024]

Summary

• restore the search URL helper behavior so empty queries do not send a blank q parameter
• keep empty-query search pages aligned with the homepage discovery requests
• update regression tests for empty-query discovery behavior

Verification

• ./node_modules/.bin/vitest run src/shared/hooks/skill-query-helpers.test.ts src/pages/search.test.tsx
• ./node_modules/.bin/vite build

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