feat: add column calculation summary row to database table view

Description

Database table views have no way to see aggregate values for a column. In Notion, clicking a "Calculate" button at the bottom of each column shows sum, average, min, max, count, etc. This is one of the most-used database features for project tracking and data analysis. Adding a summary row at the bottom of the table gives users at-a-glance insights without exporting to a spreadsheet.

Approval Required

This is a HIGH risk change because it adds a new feature surface:

• Adds a new UI element (summary row) to the table view that interacts with virtualization, column resize, and column reorder
• Requires per-column calculation config persisted in database_views.config
• Calculation logic must handle all property types correctly (sum only for numbers, count for all types, etc.)
• The summary row must stay pinned at the bottom during scroll (outside the virtualized area)

Comment "approved" to release this to the automation queue.

Acceptance Criteria

• A summary row appears at the bottom of the table view, pinned below the scrollable area
• Each column cell in the summary row shows a "Calculate" button (muted text) when empty
• Clicking "Calculate" opens a dropdown with type-appropriate options:
  • All types: None, Count all, Count values, Count unique, Count empty, Percent empty, Percent filled
  • Number: + Sum, Average, Min, Max, Median, Range
  • Date: + Earliest, Latest, Date range
  • Checkbox: + Checked, Unchecked, Percent checked
• Selecting a calculation shows the result in the summary cell
• Results update live as rows are added, edited, or filtered
• The selected calculation per column is persisted in database_views.config.column_calculations
• Each view maintains independent calculation settings
• Summary row aligns with column widths (including after resize and reorder)
• Storybook stories cover: no calculations, mixed calculations, number sum, date range
• E2E test: add number column → add rows with values → select "Sum" → verify sum displays → filter rows → sum updates → switch view → switch back → calculation persists
• pnpm lint && pnpm typecheck && pnpm test pass

Dependencies

None

Technical Notes

• Add column_calculations?: Record<string, string> to DatabaseViewConfig in src/lib/types.ts (maps property ID → calculation type)
• Create src/lib/database-calculations.ts with pure functions: calculateColumnSummary(rows, propertyId, calculationType, propertyType) — returns formatted string
• Render the summary row outside the virtualized container in table-view.tsx, below the scrollable area, using the same CSS grid template as data rows
• Use handleViewConfigChange to persist calculation selections (same pattern as column_widths, row_height)
• The summary row should use position: sticky; bottom: 0 or be placed after the scroll container
• Use a DropdownMenu for the calculation picker, grouped by category (Common, Number-specific, etc.)
• Reference .agents/conventions.md for component patterns and .agents/design.md for visual styling

Start with src/lib/types.ts, src/lib/database-calculations.ts, and table-view.tsx, then review .agents/conventions.md and .agents/design.md. Run the existing checks with pnpm lint && pnpm typecheck && pnpm test; the work is done when the summary row, persistence, live updates, Storybook stories, and specified E2E flow meet the acceptance criteria.