fix: let double-click open a table cell again

[vibhavkatre]

Double-clicking a whiteboard table cell selected the table but never left a caret in it. Two separate faults were in the way — one on every document type, one specific to the unified canvas.

Closes #353
Closes #354

#353 — the editor was thrown away immediately after being opened

onDoubleClick did this:

ui.state.editingCell = { tableId, row, col }
ui.selectTable(table.id)

selectTable routes through setSelection, which clears editingCell by design — any selection change should close an open cell editor. So the second line discarded what the first had just set, on every double-click, including when the table was already selected.

Fixed by selecting first, then opening the cell. The clearing behaviour in setSelection is correct and is unchanged.

WhiteboardTable.vue's own header comment claimed double-click was the way in, so this read as intended behaviour that had silently stopped working.

#354 — on the unified canvas the handler was unreachable anyway

delegatesSurface() only delegates to the whiteboard layer for unambiguous whiteboard tools; select falls through to shared block handling. That is deliberate for pointer events, but it also gated onDoubleClick, so on a unified document the whiteboard's double-click never ran at all.

The cell lookup is now a small exported editTableCellAt(store, point), which DiagramCanvas calls directly for the unified case — after its own block-shape check, and only when a table is actually under the cursor.

That last part is deliberate: force-delegating the whole handler would also have brought the whiteboard's "double-click empty canvas creates a text box", which the unified canvas does not do today. There is a test pinning that.

Testing

842 vitest and 8 E2E (3 new), run against a real browser.

The new unit test goes through the real useWhiteboardUi singleton rather than the fake ui the neighbouring tests use, because the bug lived in the interaction between the two. I verified it fails against the old ordering:

× editTableCellAt > leaves the cell open after selecting the table
  → selecting the table cleared the cell it just opened: expected null to deeply equal { tableId: 'wte1d5', row: 1, col: 0 }

E2E covers double-click opening a cell on both a legacy whiteboard and a unified document, typing into it and asserting the persisted document, plus the guard that double-clicking empty unified canvas still creates nothing.

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