FN-5915: show unread badge on header mailbox

Add unread mailbox status to the desktop header toggle while keeping pending approvals prioritized.

- show an unread status dot on the desktop header mailbox toggle when unread mail exists without pending approvals
- keep pending-approval indicators taking precedence and hide mailbox indicators while the mailbox view is active
- extend Header coverage for unread-only, pending-only, combined, zero-count, and active-mailbox states
- update restart integration coverage to reuse an existing worktree during orphaned resume concurrency

Files changed:
 docs/dashboard-guide.md                            |  2 +-
 packages/dashboard/app/components/Header.tsx       |  9 +++++--
 .../app/components/__tests__/Header.test.tsx       | 26 ++++++++++++++++---
 .../src/__tests__/restart.integration.test.ts      | 29 ++++++++++++++++++++--
 4 files changed, 58 insertions(+), 8 deletions(-)

Fusion-Task-Id: FN-5915

Fusion-Task-Lineage: 3a207e2c-ca58-402f-94cf-3a9e514fe263

docs/dashboard-guide.md

@@ -211,7 +211,7 @@ Mailbox view shows inbox/outbox communication threads and unread state.
 - reply rows in the mailbox modal can expand inline to show the replied-to message context for easier thread reading
 - mailbox now includes an **Approvals** tab with pending and history filters (`approved` / `denied` / `completed`), approval detail context, and inline approve/deny actions for pending requests
 - in the **Agents** tab, the agent selector now includes **All agents**, which shows one combined agent-to-agent stream (with sender + recipient labels); selecting a specific agent still shows Inbox/Outbox subtabs
-- mailbox entry points now show pending-approval indicators: Header mailbox toggle dot, Header overflow mailbox badge, Mobile mailbox tab dot, and Mobile More → Mailbox badge
+- mailbox entry points now show unread/pending indicators: the desktop Header mailbox toggle shows a pending-approval dot first or an unread dot when unread mail exists without pending approvals, while Header overflow + Mobile mailbox entry points continue to surface mailbox badges/dots
 - approval lifecycle SSE events (`approval:requested`, `approval:updated`, `approval:decided`) trigger mailbox approvals refresh without manual reload
 - when a task newly enters `awaiting-approval`, the app shows a persistent approval banner above project content with an **Open Mailbox** CTA; dismissals are remembered per approval item until that item advances or a different one arrives
 - Visible message history/threading is driven by explicit `message.metadata.replyTo.messageId` links