* auto-select newly uploaded docs/URLs in My Documents list
* fix: improve auto-select reliability and fix debounce/selection bugs
- Add missing `await` on fetchKeys in handleSendLink so loading state
and auto-select timing work correctly
- Use functional update for setSelectedItems to merge with existing
selections instead of replacing them
- Stabilize debounced fetchKeys with useRef so rapid uploads actually
debounce instead of creating independent timers per render
- Rename shadowed local variables (availableDocs -> filteredAvailableDocs)
for clarity
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Co-authored-by: Timothy Carambat <rambat1010@gmail.com>
