Fix editor sync/scroll lock between paired panels

[sampjvv]

Summary

• Add sync guard so cell clicks only navigate the source panel when sync is enabled
• Add lock/unlock toggle button that sends toggleChapterSync to the editor provider; toggling ON syncs source to the target's current chapter
• Chapter navigation arrows now send syncChapterToOther so the paired panel follows when sync is ON
• Add onDoubleClick stop-propagation on the header container to prevent accidental panel swaps

Test plan

• Webview tests pass (npm run test:webviews — 219 passed)
• Open .codex + source side-by-side, verify totalHandlers is 84
• Click lock/unlock toggle — sync should enable/disable
• Navigate chapters with arrows — paired panel should follow when sync is ON
• Click verses in target — source should only jump when sync is ON
• Rapidly double-click nav arrows — panel sizes should NOT swap

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