refactor(dashboard): graduate App.tsx off the line-count ratchet baseline

App.tsx dropped from 2,729 to 1,636 lines through the module-breakup refactor
(U1-U7), now well under the 2,000-line cap. Remove its grandfathered entry
from scripts/line-count-baseline.json so the file is subject to the cap going
forward and can never regress above 2,000. Scoped change: only the App.tsx
entry is removed; every other ceiling is untouched (a full `--update` would
have re-derived the whole baseline and re-raised ceilings for unrelated
grown files, which AGENTS.md forbids).

U8 (App.tsx module-breakup plan).

scripts/line-count-baseline.json

@@ -19,7 +19,6 @@
   "packages/core/src/mission-store.ts": 4382,
   "packages/core/src/store.ts": 16939,
   "packages/core/src/types.ts": 7269,
-  "packages/dashboard/app/App.tsx": 2729,
   "packages/dashboard/app/api/legacy.ts": 10742,
   "packages/dashboard/app/components/AgentDetailView.tsx": 5400,
   "packages/dashboard/app/components/AgentsView.tsx": 2109,