fix(ui): allow web search tests to use env keys

[snvtac]

Summary

• let the Settings web-search test route resolve provider keys the same way runtime does: inline key first, then provider-specific env/customEnv fallbacks
• allow the Settings Test button to run without an inline API key and send the current customEnv form state to the backend
• preserve masked saved customEnv secrets without reviving keys removed from the current form
• update Settings copy and add targeted backend/frontend regression coverage

Validation

• pnpm --dir ui exec vitest run server/routes/config.test.js src/components/settings/view/tabs/PilotDeckConfigTab.webSearch.test.ts
• pnpm --dir ui exec eslint src/components/settings/view/tabs/PilotDeckConfigTab.tsx src/components/settings/view/tabs/pilotDeckConfigForm.ts src/components/settings/view/tabs/PilotDeckConfigTab.webSearch.test.ts (passes with one existing warning in PilotDeckConfigTab.tsx)
• node --check ui/server/routes/config.js && node --check ui/server/routes/config.test.js
• git diff --check

Notes

Full pnpm --dir ui run typecheck still fails on existing repo-wide React/lucide type conflicts and unrelated TS errors. Full pnpm --dir ui run lint still fails on existing unrelated lint errors in chat/WebSocket/session tests; the touched Settings files have no new lint errors.

Closes #254