Sheets: add selection stats (sum, average, count) to sheets bottom bar

[the-narwhal]

What

Adds a selection statistics display to the Proton Sheets bottom bar, the sum, average, and count that appear in the bottom-right corner when you highlight multiple cells, matching the behaviour of Google Sheets, Excel, and LibreOffice Calc.

Why

This feature existed in the legacy UI via the SheetStatus component from @rowsncolumns/spreadsheet, but was never ported when the new UI was built. There was no comment or TODO indicating it was deliberately dropped, it appears to have simply been missed.

How

• Added a SelectionStats memo component to BottomBar.tsx. It reads activeSheetId, selections, merges, and getEffectiveValue from useUI((ui) => ui.legacy.*) and renders the stats on the right side of the bar.
• The useMemo dependencies include getEffectiveValue, which comes from spreadsheetState and gets a new reference after formula recalculation, so stats stay up to date without any extra wiring.
• Extracted all computation into a standalone selectionStats.ts module (no external dependencies) for clean testability.
• Gave SheetTabs a flex-1 min-w-0 class so it properly fills the available space and the stats are naturally pinned to the right.

Behaviour

• Single cell selected → nothing shown
• Multiple cells with numbers → Sum: X Average: Y Count: Z
• Mixed numbers and text → Sum and Average reflect only numeric cells; Count reflects all non-empty cells
• All-empty selection → nothing shown
• Merged cells → only the owner (top-left) cell is counted, never double-counted
• Very large selections → processing is capped at 50,000 cells to bound CPU cost
• Read-only and revision modes → stats are still shown (they're informational only)

Testing

27 unit tests in selectionStats.test.ts covering all of the above cases including edge cases for Infinity, NaN, false, 0, floating-point accumulation artifacts, multi-range aggregation, merge deduplication, the cell cap, and locale-aware number formatting.