fix(ui): isolate canvas design and interaction fixes

[SirEdvin]

Summary

• Reopens the non-OIDC fixes from the previous branch on top of current upstream main
• Keeps the PR diff focused on canvas/object creation consistency, add-object popup sizing, realtime add/remove race handling, and canvas/C4 theme alignment
• Adds a test-fixture guard so direct DB tests do not wipe the built-in technology seed through TRUNCATE ... CASCADE
• Deliberately excludes the duplicated OIDC/auth-provider commits because upstream main already has OIDC support

Verification

• npm test — 27 files / 190 tests passed locally
• npm run build — passed locally (existing large chunk warning)
• python3 -m ruff check backend/tests/conftest.py backend/app/api/v1/connections.py backend/app/api/v1/diagrams.py backend/app/api/v1/objects.py backend/app/services/diagram_service.py backend/tests/api/test_object_create_visibility.py — passed locally
• GitHub Actions on this PR: build-frontend, build-backend, and test all passed

OIDC scope check

• Branch was created from upstream main and cherry-picked only non-OIDC fixes
• git diff --name-only upstream/main...HEAD | grep -Ei 'oidc|oauth|auth' returns no changed auth/OIDC files