Add notebook management and cross-notebook search

[aliorhun]

Summary

Implements the next notebook UX package:

Notebook management

• rename a notebook by double-clicking its tab name
• right-click tab menu with Rename and Duplicate
• duplicate notebooks with collision-safe names
• drag and drop tabs to reorder them
• replace destructive close confirmation with an 8-second Undo / Geri Al toast
• preserve active notebook identity while tabs are reordered

Cross-notebook search

• add Search / Ara to the top bar
• open search with Ctrl/Cmd + K
• search both notebook names and every notebook line
• show notebook name, line number and content snippet
• jump directly to the matching notebook/line in CodeMirror
• Enter opens the first result; Escape closes search
• cap results at 100 to keep the UI responsive

Validation/docs

• extend the offline verifier to assert the management/search UI and source wiring
• document the new notebook management actions and shortcut

All notebook changes continue to persist through the existing localStorage/save pipeline and work offline.