fix(ui): generate fumadocs-mdx collections/* via postinstall

[oktofeesh1]

Summary

• apps/loopover-ui/tsconfig.json maps collections/* to ./.source/*, a directory fumadocs-mdx's Vite plugin generates from source.config.ts. .source/ is gitignored and nothing generated it outside of an actual vite dev/vite build run.
• npm run ui:typecheck (and therefore CI's validate-code/validate jobs) runs bare tsc --noEmit with no prior Vite invocation, so on a genuinely fresh checkout collections/browser/collections/server fail to resolve (TS2307: Cannot find module).
• This has been broken since the fumadocs-mdx docs migration landed in PR #6271 — that PR's own CI failed for this exact reason. Every PR touching apps/loopover-ui since inherits the same failure on a fresh CI runner, even when npm run test:ci passes locally on a working tree that has ever run vite dev/build before (which leaves .source/ on disk).
• Fix: wire fumadocs-mdx's own generator (node_modules/.bin/fumadocs-mdx, its documented postinstall entry point) as apps/loopover-ui/package.json's postinstall script, so npm ci always regenerates .source/ before any other script runs.

Scope

• The PR title follows type(scope): short summary Conventional Commit format.
• This PR is focused: a 2-line fix (one package.json script, the corresponding package-lock.json metadata bit) isolated from any content/feature work.
• This follows CONTRIBUTING.md and does not reintroduce GitHub Pages, VitePress, site/, or CNAME.
• Closes fix(ui): fumadocs-mdx collections/* virtual modules missing on a fresh checkout #6304

Validation

• git diff --check
• npm run actionlint
• npm run typecheck
• npm run test:coverage locally — apps/loopover-ui/** only, outside coverage.include; codecov/patch does not apply
• npm run test:workers
• npm run build:mcp
• npm run test:mcp-pack
• npm run ui:openapi:check
• npm run ui:lint
• npm run ui:typecheck
• npm run ui:build
• npm audit --audit-level=moderate
• Reproduced the exact CI failure locally and confirmed the fix: rm -rf apps/loopover-ui/.source && npm ci --prefer-offline --no-audit --no-fund (the literal command CI runs) regenerates .source/ via the new postinstall hook, and npm --workspace apps/loopover-ui run typecheck then passes — it failed with the identical TS2307 errors before this fix.

Safety

• No secrets, wallet details, hotkeys, coldkeys, user PATs, private keys, raw trust scores, private rankings, or private maintainer evidence are exposed.
• Public GitHub text stays sanitized, low-noise, and does not imply compensation guarantees or optimization tactics.
• Auth, cookie, CORS, GitHub App, Cloudflare, or session changes include negative-path tests. — N/A
• API/OpenAPI/MCP behavior is updated and tested where needed. — N/A
• UI changes use live API data or real empty/error/loading states, not production mock/demo fallbacks. — N/A, build-tooling fix only, no UI behavior changes
• Public docs/changelogs are updated where needed; changelogs are only edited for release-prep PRs.

Notes

• No UI Evidence table — this is a build-tooling fix with zero rendered-output changes.
• Unblocks every pending/future PR touching apps/loopover-ui, including the AMS docs-porting sub-issue PRs currently being worked (docs(ams): port DEPLOYMENT.md to a website docs page (docs.ams-deployment.tsx) #6022, docs(ams): port docs/operations-runbook.md to a website docs page (docs.ams-operations-runbook.tsx) #6023, ...).

[superagent-security]

Superagent didn't find any vulnerabilities or security issues in this PR.