Clarify invalid filters and route titles

[mattfaltyn]

Summary

• Distinguish invalid filter entries from applied thresholds in the chain toolbar and chips. Valid thresholds retain their own count; invalid entries show Fix N and remain clearable.
• Explain an inverted DTE range directly in the empty state while preserving its existing filter behavior.
• Set descriptive browser titles for the option chain, watchlist, and unknown routes; document the filter feedback in the README.

Verification

• scripts/verify-fast: 376 backend and 131 frontend tests passed.
• scripts/verify: local tests, audit, lint, API generation, and build passed; Playwright could not bind the user-owned backend on port 8000.
• Exact-head CI passed the full gate, including Playwright and packaged resources, on 5c817fd.
• Narrow, tablet, and desktop layouts were inspected in light and dark themes; no document overflow or console errors were observed.

Pad: WORK-88.