Fix cross-block text selection

Summary

When selecting text across multiple Seeds (blocks), the selection should work seamlessly like a regular text editor. Currently, text selection within a single Seed works, but selecting across block boundaries does not.

Expected behavior

• Click and drag from one block into another should select text across both
• Shift+click in a different block should extend text selection
• Selected text across blocks should be copyable as continuous text

Current behavior

Multi-block selection via Shift+Arrow works for block-level operations (highlighting entire blocks), but cross-block text selection (partial text from multiple blocks) is not implemented.

Technical notes

• Plots handles block-level selection via useSelectionStore
• Seeds handle text editing via contenteditable
• May need to coordinate between Seed's text selection and Plots' block selection
• Consider how this interacts with formatting operations on selected text

Related

Part of the v0.6 CodeMirror block rewrite. Lower priority than core editing functionality.