fix(mobile): distinguish unknown presence on identity surfaces

[loganj]

🤖

Summary

When mobile didn't know someone's presence — right after opening the app, or when the relay simply had no data — DM headers, DM-list avatars and profiles showed that person as Offline, with the same gray dot as someone genuinely offline. This PR separates "we don't know" from "we know they're offline": unknown presence now reads Unknown with no dot or notch, while confirmed Online/Away/Offline look exactly as before.

• Applies to the DM header, DM-list avatars, and the profile sheet. Avatar geometry is unchanged — the unconfirmed indicator is hidden, not reshaped.
• Confirmed Online/Away/Offline presentation is untouched.
• The profile exposes one presence accessibility label, with no duplicate screen-reader announcements.

Related issue

• Fixes: N/A. No separate issue; the related work is the stack below.
• Depends on fix(mobile): hydrate and refresh relay presence snapshots #7382 (declared base), which supplies the presence data — this PR is the visible half of that change.
• Draft — not requesting merge yet; the security advisory doesn't run on this stacked base (it reviews main-based ranges).

Testing

• Widget regressions cover unknown/online/away/offline transitions, fixed avatar bounds, DM-list colors, and profile semantics.
• At the branch head: just mobile-check and the full mobile test suite pass independently, and every broader CI gate has successful evidence after isolated recovery runs (no monolithic local just ci run) — receipts in the exact-head evidence comment.
• Verification is widget-test level; no native device or simulator run is claimed.

To see it: open a DM whose presence isn't confirmed yet (e.g. right after a cold open) — the header and profile now read Unknown instead of Offline.

Screenshots

Flutter production-widget test renders — not native-device screenshots or acceptance captures.

Scenario	Before	After
Profile sheet for a person whose presence isn't confirmed yet (e.g. right after a cold open)

Confirmed Online/Away/Offline presentations are unchanged by this PR.

Capture provenance

Rendered by the Flutter widget engine in a flutter test run (production widgets, production theme; no device or simulator). Before: this PR's declared base a8e8028bfb9bb67de2db03ed448839e2f3ab2b0d. After: its head dd955a0c2ac80ada39b416a2f8b7481afb8773bf.