fix(editor): restore unmerge option + in-editor merge confirmation (#691)

[sampjvv]

What & why

Fixes #691 — "merged content no longer shows the option to unmerge" — plus a UX follow-up on the merge confirmation.

1. Unmerge button no longer showed (the reported bug)

In Source Editing (correction) mode the provider intentionally keeps merged cells in the translation units so they can be unmerged, and CellContentDisplay renders a Cancel merge button on them. But CellList was filtering merged cells out in exactly that mode (filteredTranslationUnits), so the merged cell never reached CellContentDisplay and the button could never render.

The filter contradicted the rest of CellList, which already styles merged cells (greyed at opacity: 0.5, ❌ line number). Removing it restores the intended behavior; it's safe because the provider already strips merged cells in every other mode.

2. Merge confirmation moved into the editor

Confirming a merge used a native vscode.window.showWarningMessage, which pops up in the bottom-right corner of the screen — far from the cell — and wrongly said the action "cannot be undone" (merges are reversible via unmerge).

It's now a small in-editor modal (the same Dialog pattern as the existing auth/offline modals) with reversible wording and Cancel / Merge buttons. confirmCellMerge is only posted once the user confirms; the handler no longer shows the native dialog (its child-cell guard is preserved), and the now-unused message payload field is removed from the type.

Note: per the ticket, unmerge intentionally only re-exposes the cell (sets merged: false); it does not reverse the content concatenation. That behavior is unchanged here.

Changes

• webviews/.../CodexCellEditor/CellList.tsx — remove the merged-cell filter.
• webviews/.../CodexCellEditor/CellContentDisplay.tsx — in-editor merge-confirmation modal.
• src/providers/codexCellEditorProvider/codexCellEditorMessagehandling.ts — drop the native confirmation from confirmCellMerge.
• types/index.d.ts — remove the unused message field from confirmCellMerge.
• Tests: CellList.mergedCellUnmerge.test.tsx, CellContentDisplay.mergeConfirm.test.tsx.

Testing

• Webview suite: 160 passed / 2 skipped (19 files), including the 2 new tests.
• ESLint: 0 new warnings; extension typecheck clean for the changed files.
• Manual: Source Editing Mode → merge a non-first cell → in-editor modal appears (not the screen corner); Cancel aborts, Merge proceeds and the merged cell stays greyed with a working unmerge button.

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