Add editor undo and redo controls

[danielss-dev]

What changed

• Added state-aware Undo and Redo controls to the Work Content toolbar.
• Bound the controls directly to Pierre's structure-aware editor history, shared with Mod+Z, Mod+Shift+Z, and Ctrl+Y.
• Reset the exposed history handle when Strand intentionally rebuilds the editor after Discard or a clean external refresh.
• Added localized labels, Strand-native icons and styling, plus README, roadmap, task, learning, and user-guide updates.

Why

Pierre already provided undo/redo through its editor keymap, but Strand did not expose discoverable pointer controls or their availability state. This adds the visible affordance without maintaining a parallel history stack.

User impact

Users can now undo and redo Work Content edits from the toolbar or keyboard. The buttons disable automatically at history boundaries and preserve Strand's existing session-draft and stale-write behavior.

Validation

• pnpm --filter ./ui exec tsc --noEmit
• pnpm --filter ./ui test — 337 tests passed
• pnpm --filter ./ui build
• cargo build --no-default-features
• Live Tauri/WebView2 pass covering typing, toolbar Undo/Redo, Ctrl+Z/Ctrl+Y, disabled states, and final draft cleanup