Desktop web: legacy /new?modal=add-custom-connector install links silently ignored; add-connector modal's exit navigation still routes to legacy /settings/connectors tombstone

## Summary

Two desktop-web regressions around custom-connector install links ("Add to Claude" deep links), observed 2026-08-27, macOS Chrome, signed in:

1. The **legacy install-link pattern** `https://claude.ai/new?modal=add-custom-connector&connectorName=NAME&connectorUrl=ENCODED_URL` is now **silently ignored** — the page renders a normal new-chat screen, no dialog, no error. Many published "Add to Claude" buttons in the wild use this pattern (it was the working pattern before the Customize move).
2. The **current pattern** `https://claude.ai/customize/connectors?modal=add-custom-connector&…` opens the dialog correctly (prefilled, external-link warning shown) — but the dialog's exit navigation still targets the **legacy** route: clicking **Cancel** lands on `https://claude.ai/settings/connectors?connectorName=…` which renders only the "Connectors have moved to Customize" tombstone. On 2026-08-26 we observed the same tombstone-bounce after clicking **Continue** on the then-served flow, which killed the add entirely (manually adding the identical values through Customize → Add worked).

## Steps to reproduce

**Repro 1 — legacy pattern inert**
1. Desktop Chrome, signed in to claude.ai.
2. Open `https://claude.ai/new?modal=add-custom-connector&connectorName=Example&connectorUrl=https%3A%2F%2Fexample.com%2Fmcp`
3. Observe: normal new-chat page. No add-connector dialog, nothing prefilled, no error.

**Repro 2 — modal exits route to the legacy tombstone**
1. Desktop Chrome, signed in.
2. Open `https://claude.ai/customize/connectors?modal=add-custom-connector&connectorName=Example&connectorUrl=https%3A%2F%2Fexample.com%2Fmcp`
3. Dialog opens prefilled (works).
4. Click **Cancel** → browser navigates to `/settings/connectors?connectorName=…` and renders the "Connectors have moved to Customize" stub. Expected: stay on Customize → Connectors.
(On 2026-08-26, **Continue** exhibited the same bounce-to-tombstone on the then-active flow, killing the add. We did not re-click Continue on the customize path today to avoid creating a connector, so its current behavior is unverified — but it appears to share the same stale route table as Cancel.)

## Expected

- The legacy `/new?modal=add-custom-connector` pattern either keeps working or redirects to the customize equivalent with params preserved (in-the-wild buttons depend on it).
- All dialog exits (Cancel / Continue / post-add) navigate within `/customize/connectors`, never to the retired `/settings/connectors` route.

## Environment

macOS, Chrome (desktop web), claude.ai, signed in, 2026-08-27.

## Related

#895 documents the iOS-Safari and signed-out legs of install-link breakage for the current pattern; this report covers the desktop-web legacy-pattern regression and the stale exit routing.