fix: keep caret in place when editing empty table cells

[lastday97]

Why

While editing an inline-rendered table cell, the caret snaps back to the
start of the cell whenever the debounced updateBlock re-renders the block
renderer — worst when the cell is empty. (Closes #3)

Root cause

Cells were rendered with a React children prop. Every updateBlock echo
re-run reconciled the tree against freshly-written content; React's direct
text-child fast path (setTextContent) rewrote the live contenteditable
text (empty cells rebuild the text child via node.textContent = ''),
resetting the caret. The old "identical text is a no-op" assumption broke
for empty cells.

Fix

Render <th>/<td> WITHOUT a children prop so React never owns cell text
(shouldSetTextContent stays false — no more setTextContent during
re-render). Initial text is applied once via a cell ref (syncCellText),
guarded by a textContent comparison so the normal echo re-render is a
no-op and the caret is left untouched. Structural ops still overwrite
genuinely different text; resumePinnedToolbar re-anchors the caret.

Verification

• New unit tests for syncCellText (no-op on equal text, write/clear/overwrite,
  null-safety) — npm test 10/10 pass.
• npm run build succeeds.
• Manually verified in Logseq: caret stays put while typing mid-cell and
  while clearing a cell.