App: repeater manage sheet polish

[MrAlders0n]

Small things in lib/widgets/repeater_admin_sheet.dart that are all one or two lines each:

• A failed Unclaim shows the generic "The server rejected the request." even when the provider handed back a specific sentence. Use result.message ?? result.userMessage, the way Claim and Upload already do.
• A failed upload's red sentence survives "Fetch again". Clear _uploadResult when a new fetch starts.
• A failed Unclaim wipes the "Claimed. Listed as ..." line while the button correctly stays Unclaim. Only clear _claimResult on success.
• "All N neighbours loaded" also shows when Load more stopped for the 30 page brake or an empty page, where held is less than the repeater's total. Say "N of M loaded" in that case.
• Log in and Reset route only look at session.busy, so they stay tappable while a claim or upload POST is in flight. A login there flips the state under the pending result. Gate them on _serverBusy too.
• The proven neighbour row on the map's detail sheet has no gap between the name and the SNR, so a long name butts against the dB value. The My Repeaters subtitle separates the prefix and the region with two spaces, which most layouts collapse; a middot reads better.
• Unresolved proven neighbours always render "(unknown)". repeaterNameForPrefix exists and could name some of them from the local list.