FN-6142: fix mobile visibility restore scroll anchoring

Restore pinned mobile chat scroll position when visibility-driven refetch hooks fire.

- re-anchor mobile chat to the bottom during visibility/pageshow refetch capture when the user was pinned
- tighten ChatView visibility restore regression tests to assert the synchronous re-anchor path directly
- remove the ChatView rooms test quarantine and restore the dashboard include list entry

Files changed:
 packages/dashboard/app/components/ChatView.tsx          |  6 +++++-
 .../app/components/__tests__/ChatView.rooms.test.tsx    | 17 ++++++++---------
 .../app/components/__tests__/ChatView.test.tsx          |  7 ++++---
 packages/dashboard/vitest.config.ts                     |  2 +-
 scripts/lib/test-quarantine.json                        |  8 +-------
 5 files changed, 19 insertions(+), 21 deletions(-)

Fusion-Task-Id: FN-6142

Fusion-Task-Lineage: 2974cf0d-5512-4b24-ab0c-2d2fb63fd41a

scripts/lib/test-quarantine.json

@@ -1,10 +1,4 @@
 {
   "$comment": "Flaky-test quarantine ledger (deletion ratchet — see AGENTS.md 'Flaky tests: quarantine on sight' and docs/testing.md 'Quarantine ledger and the deletion ratchet'). A test observed failing without a corresponding real bug is quarantined ON SIGHT: add an entry here AND a matching one-line `exclude` entry in that package's vitest config, in the same commit. Every entry needs a non-empty `reason` (link the failing run) and a `quarantinedAt` date — the entry expires 14 days later, at which point the test file is DELETED unless someone rescues it with evidence it catches real regressions plus a root-cause fix (never appeasement). There is deliberately no loader module and no automation around this file: it is a dated record, the vitest config exclude is the mechanism, and the sweep is policy executed by whoever touches the suite.",
-  "entries": [
-    {
-      "file": "packages/dashboard/app/components/__tests__/ChatView.rooms.test.tsx",
-      "reason": "FN-6103 workspace pnpm test observed flaky mobile visibility restore assertion: expected scrollTop 1180, received 300 in ChatView.rooms.test.tsx:805; unrelated to CSS-only composer button sizing change.",
-      "quarantinedAt": "2026-06-09"
-    }
-  ]
+  "entries": []
 }