refactor: move bookmark glyph into shared icon

[zerone0x]

Summary

• add BookmarkIcon to the shared icon module
• replace the duplicated inline bookmark SVGs in the terminal bookmark gutter
• keeps the two existing rendered sizes (15px and 14px) while removing the local pasted path

Refs #208

Tests

• npm ci --ignore-scripts (plain npm ci failed because node-pty native postinstall could not rebuild in this container)
• npm run typecheck
• git diff --check

[johannesjo]

Thank you very much! <3