Conversation visibility: harden client-side edge cases

[beastoin]

Follow-up from #5268 (conversation visibility toggle).

Current Behavior

The visibility toggle works for the happy path, but a few client-side edge cases aren't covered yet.

Expected Behavior

Robust handling of failures and consistent backend persistence.

Affected Areas

File	Description
app/lib/pages/conversation_detail/page.dart	Share button sets visibility locally but doesn't call setConversationVisibility() API
app/lib/pages/conversation_detail/widgets/share_to_contacts_sheet.dart	Same — local-only update
app/lib/pages/conversation_detail/widgets.dart	No error feedback (snackbar) on API failure; no loading/disabled state during async PATCH

Solution

• Call setConversationVisibility() in the share button and contacts sheet flows so visibility persists after app restart
• Show a snackbar when the PATCH fails so the user knows the toggle didn't stick
• Disable the chip or show a spinner while the API call is in flight to prevent double-taps

Impact

None for existing users — these are improvements on the new visibility feature from #5268.

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by AI for @beastoin