fix: resolve compiler warnings across 6 files

[datlechin]

Summary

• Replace deprecated String(bytesNoCopy:length:encoding:freeWhenDone:) with UnsafeBufferPointer + String(decoding:as:) in SSH keyboard-interactive auth
• Change var root to let root in RedisKeyTreeViewModel (reference is never reassigned)
• Add explicit self. for sidebarViewModel in MainContentCoordinator+Navigation closure
• Remove unused connectionName variable in MainContentView
• Wrap main-actor-isolated calls in MainActor.assumeIsolated in DataGridView notification observers (already dispatched on queue: .main)
• Remove redundant as? InMemoryRowProvider casts in DataGridView+RowActions (type already known)

Test plan

• Build succeeds with zero warnings
• SSH keyboard-interactive authentication still works
• Redis key tree loads correctly
• DataGrid row copy/paste operations unchanged