FN-8054: add pinned chat conversations

Add durable, scoped pinning for Direct chat conversations.

- Add pinned session persistence, migration coverage, and archive-safe row locking.
- Enforce a three-conversation per-project pin limit through the chat API.
- Add desktop and mobile pin controls, sorting, indicators, and regression tests.

Files changed:
 .changeset/fn-8054-pin-conversations.md            |  7 ++
 docs/dashboard-guide.md                            |  2 +
 .../postgres/satellite-db-injected-stores.test.ts  | 13 ++++
 packages/core/src/async-chat-store.ts              | 27 ++++++++
 packages/core/src/chat-store.ts                    | 63 ++++++++++++++++--
 packages/core/src/chat-types.ts                    |  9 +++
 .../core/src/postgres/migrations/0000_initial.sql  |  1 +
 .../postgres/migrations/0012_chat_session_pins.sql |  8 +++
 packages/core/src/postgres/postgres-health.ts      |  3 +
 packages/core/src/postgres/schema-applier.ts       | 30 ++++++++-
 packages/core/src/postgres/schema/project.ts       |  3 +
 packages/dashboard/app/api/legacy.ts               |  1 +
 packages/dashboard/app/components/ChatView.css     | 32 +++++++++-
 packages/dashboard/app/components/ChatView.tsx     | 74 ++++++++++++++++++++--
 .../dashboard/app/hooks/__tests__/useChat.test.ts  | 21 ++++++
 packages/dashboard/app/hooks/useChat.ts            | 72 +++++++++++++++++----
 .../dashboard/src/routes/register-chat-routes.ts   | 25 +++++++-
 17 files changed, 366 insertions(+), 25 deletions(-)

Fusion-Task-Id: FN-8054

Fusion-Task-Lineage: 088cb01c-582b-4f56-a222-214da90ff356

Co-authored-by: Fusion (runfusion.ai) <noreply@runfusion.ai>

packages/dashboard/app/api/legacy.ts

@@ -9987,6 +9987,7 @@ export function updateChatSession(
     modelId?: string | null;
     agentId?: string;
     thinkingLevel?: string | null;
+    pinned?: boolean;
   },
   projectId?: string,
 ): Promise<ChatSessionResponse> {