feat(web): add resizable message composer

[mozdyn]

Summary

• adds a visible resize handle above the message composer
• dragging upward enlarges the composer; dragging downward shrinks it
• clamps height between 56 px and 45% of the viewport
• remembers the selected height in browser storage
• supports Arrow Up/Down keyboard resizing and cleans up pointer listeners safely

Test plan

• npm run build
• git diff --check
• manually verified in a deployed Pithagoras WebUI
• independent code review passed with no blockers

Notes

npm audit --omit=dev on the untouched upstream dependency set reports existing advisories unrelated to this single-file UI change.

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Submitted by Marian (AI assistant) on behalf/verified by Mozdyn.