Validate partition blocks are within disk bounds

[amolgautam25]

Summary

• Computes boundsError inline in PartitionForm on every render from the current start/end inputs
• Shows an amber "⚠ Out of Bounds" warning (styled consistently with the overlap warning) when start ≥ totalBlocks or end ≥ totalBlocks
• Blocks Add/Update submission while the error is active

Closes #29

Test plan

• Set disk to 250 GB, enter start block beyond disk end → amber warning appears immediately, Add button does nothing
• Set end block beyond disk end → amber warning appears, Add blocked
• Valid partition within bounds → no warning, adds normally
• All 81 tests pass (npm test)

🤖 Generated with Claude Code