Add undo/redo with history timeline to conflict resolver UI

[Copilot]

Adds undo/redo for all state-mutating actions in the conflict resolver web UI: branch selection, delete cell, accept-all, content editing, cell drag-drop, row reordering, and checkbox toggles.

Core: useUndoRedo hook (src/web/client/useUndoRedo.ts)

• Snapshot-based undo/redo timeline with deep cloning, max 100 entries
• Exposes pushAction, undo, redo, jumpTo for arbitrary history navigation

UI: Header controls

• SVG undo/redo buttons + dropdown toggle in the header bar
• History dropdown listing all actions with labels, relative timestamps, and active/undone visual states
• Click-outside-to-close behavior

Keyboard shortcuts

• Ctrl+Z / Ctrl+Shift+Z / Ctrl+Y, skipped when focus is in textarea/input
• Gated by new mergeNB.ui.undoRedoKeyboardShortcuts setting (default: true)

Undoable actions

• Per-row: Use Base/Current/Incoming, Delete Cell
• Bulk: Accept All Base/Current/Incoming
• Content editing (debounced — captures pre-edit snapshot before first keystroke in each typing burst)
• Cell drag-drop between rows, row reorder drag-drop
• Checkbox toggles (renumber execution counts, mark as resolved)

Settings pipeline

package.json → settings.ts → resolver.ts → webTypes.ts → WebConflictPanel.ts → client types.ts → App.tsx → ConflictResolver prop

Screenshot

Original prompt

For all possible tasks in the conflict resolver web UI, create an undo/redo action. These will be clickable buttons on the header, as well as bind it to ctrl z and ctrl shift z by default (add a setting)

A (non exhaustive, probably) list of such actions are: picking a branch/delete, taking all of one branch, switching branches, etc. Check this out for yourself, and make sure the list is complete before you are done. You are expected to carefully parse through the code in order to make sure this is true.

I suggest you simply make some kind of history/timeline dropdown from the header. Keep this in the same style as the rest of the UI.

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[Avni2000]

Closing, didn't work very well at first try. Second iteration is in #50