feat: add undo/redo toolbar buttons and fix temporal partialize

[randroid88]

Summary

• Add Undo/Redo toolbar buttons with proper disabled state, wired to the zundo temporal store
• Fix temporal partialize config to only snapshot editing state (words, deletedRanges, segments, language)

Before

Undo/redo only worked via keyboard shortcuts (Ctrl+Z / Ctrl+Shift+Z) with no visual indication of whether undo/redo was available. The temporal store captured all state changes including playhead position, hover state, and selection, causing spurious undo steps when the user simply moved the playhead.

After

Undo/Redo buttons visible in the toolbar:

• Enabled when there are past/future states to navigate
• Disabled (grayed out) when the history is empty
• Only editing operations (word deletions, range changes) create undo steps

Files changed

• frontend/src/App.tsx — Add Undo2/Redo2 toolbar buttons, subscribe to temporal store
• frontend/src/store/editorStore.ts — Add partialize config to temporal middleware

Test plan

• Delete some words → Undo button enables → click Undo → words restored
• Click Redo → words deleted again
• Move the playhead, hover over words → should NOT create undo steps
• Keyboard shortcuts (Ctrl+Z, Ctrl+Shift+Z) still work alongside buttons
• Undo button disabled when no history, Redo disabled when no future states