polish(web): draw compose brief resize grip as two muted ticks

[AlexsanderHamir]

Summary

• Replace the compose brief corner mark with two parallel 45° ticks (inner slightly longer, outer shorter) so it reads as a native SE resize handle rather than a single slash.
• Color the grip with --text-tertiary via currentColor, inset it a few pixels inside the rounded card, and leave drag/min-height behavior unchanged.

Test plan

• .\scripts\check-web.ps1 -Group test-task-components (258 passed locally)
• .\scripts\check-web.ps1 -Group lint (passed locally)
• On compose v2, confirm the bottom-right brief corner shows two muted gray ticks inside the card, not a black slash on the border
• Drag the grip: editor height still changes; cannot go below 320px

Made with Cursor