fix(desktop): explain missing pairing endpoint

[kiranmagic7]

Summary

• Preserve whether the pairing URL came from the legacy /pair fallback.
• Replace only that fallback's HTTP 404 with an actionable error naming the endpoint and the two supported remedies.
• Strip query parameters and fragments before showing the endpoint; keep every other WebSocket error unchanged.

Related issue

Fixes #3779.

Deployment-side gaps are covered separately by #3627 for Compose and #3424 for chart-managed routing.

Testing

• cargo test --manifest-path desktop/src-tauri/Cargo.toml pairing_relay_tests -- --nocapture — 9 passed
• pnpm --dir desktop build:e2e && pnpm --dir desktop exec playwright test --project=smoke --grep "legacy pairing 404 explains how to configure the relay" — 1 passed
• just ci — passed

Before/after screenshots are included in a follow-up comment.

[kiranmagic7]

Screenshots

Before	After

[Chessing234]

helpful error. one tweak — if pairing_relay_url is set but unreachable, keep that distinct from the legacy /pair 404 so people dont chase the wrong fix

[kiranmagic7]

Good catch, thank you. The legacy path was already tracked separately, but configured relay failures still fell back to the generic WebSocket error. I have now made the distinction explicit and added regression coverage, so an unreachable pairing_relay_url points users to the configured endpoint rather than the /pair remedy.

[kiranmagic7]

Re-checked this after the configured-relay split landed. Head e07384c3b3f0602927330113bd8298c75ea58b0c is still mergeable; DCO, Semgrep, and zizmor are green. The remaining Buzz Actions run is https://github.com/block/buzz/actions/runs/30633199987, still action_required with 0 jobs, so there is no failing CI log for me to fix. Review focus is just the two error paths in desktop/src-tauri/src/commands/pairing.rs and the e2e coverage in desktop/tests/e2e/mobile-pairing-qr.spec.ts.