feat: cross-section cursor navigation in lyric textareas

[GabrielBG0]

Summary

• Pressing ArrowUp on the first line of a section moves the cursor to the previous section's textarea
• Pressing ArrowDown on the last line of a section moves the cursor to the next section's textarea
• Pressing ArrowLeft at position 0 jumps to the end of the previous section
• Pressing ArrowRight at the last character jumps to the start of the next section
• Column position is preserved across sections for ArrowUp/Down, matching the native cursor behavior within a single textarea

Test plan

• Open a song with multiple sections
• Place cursor on the first line of a section and press ArrowUp, verify focus moves to the previous section with the same column offset
• Place cursor on the last line of a section and press ArrowDown, verify focus moves to the next section's first line at the same column
• Press ArrowLeft at the very start of a section, verify cursor jumps to the end of the previous section
• Press ArrowRight at the very end of a section, verify cursor jumps to the start of the next section
• Verify Shift/Alt/Ctrl/Meta held down does not trigger cross-section navigation
• Verify first section ArrowUp and last section ArrowDown do nothing (no section to navigate to)
• Verify column is clamped when the target line is shorter than the current column

🤖 Generated with Claude Code