feat(dm): propagate inbox-unavailable signal to ConversationPage and nav badges

[realmeylisdev]

Spun off from #312 (trace findings in docs/brainstorm/2026-05-02-pr-312-dm-decrypt-surface-review-brainstorm.md).

Problem

#312 surfaced the bunker-decrypt-failure signal at MessagesPage only. Two other DM entry points still fail-silent:

• src/pages/ConversationPage.tsx:189 reads useDmConversation but never useDmInboxStatus. Deeplink to /messages/<id> while bunker is broken renders an empty conversation.
• src/components/AppHeader.tsx:35 and src/components/AppSidebar.tsx:69 read useUnreadDmCount, which silently returns 0 when the bunker is broken.

Acceptance criteria

• ConversationPage consumes useDmInboxStatus() and renders an inline banner (same copy as <InboxUnavailableState /> in MessagesPage.tsx:71-91) when status is 'unavailable'.
• useUnreadDmCount exposes { data, isUnavailable } so consumers distinguish zero-unread from unreadable inbox.
• AppHeader and AppSidebar Messages nav item shows an alert dot (vs. unread-count badge) when isUnavailable.
• Tests cover the unavailable state at all three surfaces.

Out of scope

• Toast/notification when status flips mid-session.
• Send-side path — covered by feat(dm): healthcheck NIP-44 capability at signer attach #311's healthcheck.

[realmeylisdev]

@jalcine — surfacing this one too: the trace during the #312 review showed the unavailable signal lands at MessagesPage only. This issue covers extending it to ConversationPage and the nav badges so the full DM surface fails-loud, not just the inbox.