feat(ledger): add keyboard-driven row selection and range-copy (Closes #466)

[laurentketterle-hub]

Summary

Implements keyboard-driven row selection and range-copy for the Ledger table as requested in Issue #466.

Features

• ✅ Click to select/deselect individual rows
• ✅ Shift+click for range selection between anchor and target
• ✅ Ctrl+A to select all visible rows on the current page
• ✅ Ctrl+C to copy selected rows as TSV (tab-separated values) with headers
• ✅ Arrow keys navigation with Shift for range extension
• ✅ Escape to clear all selection
• ✅ Visual feedback: blue highlight (bg-blue-50) with ring indicator on selected rows
• ✅ Selection count badge in header with "Copied!" feedback on Ctrl+C
• ✅ Full keyboard accessibility: tabIndex={0}, aria-selected, data-row-id
• ✅ Click on expand/collapse buttons does NOT trigger row selection

Changes

• src/pages/Ledger.tsx: +169 lines (selection state, keyboard handler, visual styling, TSV copy)
• src/pages/Ledger.test.tsx: +167 lines (12 new keyboard selection tests)

Test Results

Tests: 59 passed (59)
- All 47 existing tests still pass
- 12 new keyboard selection tests covering:
  click select, click deselect, multi-select, selection count,
  Ctrl+A select all, Escape clear, Ctrl+C copy TSV,
  no-copy when empty, "Copied!" feedback, Shift+click range,
  blue highlight styling

Verification

npm install && npx vitest run src/pages/Ledger.test.tsx

[laurentketterle-hub]

I claim this bounty. PR ready for review.