fix(files): prevent selection reset during keyboard navigation

[dschmidt]

Summary

• Keyboard arrow navigation used resetSelection() + await nextTick() + addSelection(), causing selectedIds to briefly become empty between the reset and the new selection
• This empty state caused sidebar panel components (like the maps location panel) to be destroyed and recreated, since their isVisible check failed when items was momentarily empty
• Mouse clicks used toggleSelection() which swaps atomically and didn't have this issue
• Replace the reset+add sequence with a single setSelection([nextId]) call, which atomically replaces the selection without an intermediate empty state

Test plan

• Unit tests verify that selectedIds is never empty during keyboard navigation
• Navigate files with arrow keys while the sidebar is open — panels should not flicker/recreate
• Navigate files with arrow keys — selection should move correctly
• Shift+arrow key multi-selection still works as expected

[JammingBen]

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