hud: narrow corner-label hit region to visible text width
Replaces the full-screen hitbox with one sized to the actual text box:
x starts where the text begins (after the share-area pad), w matches
currentHUDTextBoxWidth. Vertical extent still reaches y=0 so top-edge
taps register.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>