fix: use WALL_MIN_LENGTH constant to prevent ghost walls in sidebar

[thiago-lopes-dev]

Fixes #103

Problem

WALL_MIN_LENGTH was set to 0.01 (1cm), but the grid snaps in steps
of 0.5m. When a user placed a wall end point too close to the start,
the snapped points would coincide, creating a wall node with effectively
zero length that still passed the minimum length check and got persisted
to IndexedDB — causing the ghost entry in the sidebar and a crash on reload.

Changes

• Increased WALL_MIN_LENGTH from 0.01 to 0.5 in wall-drafting.ts
• Replaced hardcoded 0.01 * 0.01 checks in wall-tool.tsx with
  WALL_MIN_LENGTH * WALL_MIN_LENGTH so all validations stay in sync

Related

Closes #103