feat: scope deep links by username

[wilhel1812]

Summary

• Change path deep links to /<username>/<simulation>/<selection> and update docs/tests.
• Resolve deep-link API slug lookups inside the owner username namespace.
• Allow duplicate Simulation names across owners while rejecting duplicates for the same owner, and reject case-insensitive duplicate usernames on profile updates.

Verification

• git diff --check
• Local npm test / npm run build not run: no JS package manager is available in this shell (npm, pnpm, yarn, bun are not on PATH). CI is the verification gate for this PR.

Closes #813 after staging verification and sign-off.