Harden auth UI and E2E proxy smoke check

[Squirbie]

Summary

• stop rendering the default administrator password literal in the Auth tab
• make the E2E workflow install pnpm before using pnpm commands
• update the E2E smoke check to hit the real MediaMTX proxy route instead of missing /api/streams
• add regression assertions to the existing smoke test for these cases

Proof

• corepack pnpm test
• corepack pnpm exec tsc --noEmit
• corepack pnpm build
• git diff --check

/claim #4