fix: render volume sliders on mobile/touch devices (iOS)

[birme]

Summary

• Fixes Mobile devices missing volume sliders #621 where volume sliders were missing on iOS phones and iPads
• Root cause: the line/PGM volume slider in src/components/production-line/user-controls.tsx was gated behind !isIOSMobile && !isIpad, hiding it on all touch devices
• Removed those device conditions (and the now-unused bowser import) so the slider renders on mobile/touch as well
• Kept the unrelated program-output-line guard intact
• Added a regression test user-controls.test.tsx

Test plan

• Tests pass (npm test) — 322 tests
• TypeScript compiles (npm run typecheck)
• Lint clean (npm run lint)
• Verify volume sliders now appear for lines and PGM on an actual iOS device (manual)

Closes #621

🤖 Generated with Claude Code

[birme]

code-reviewer verdict: LGTM (automated self-review; recorded as a marker because GitHub blocks state-bearing self-review when author and reviewer are the same account).

Small, well-scoped fix for #621. Removing the !isIOSMobile && !isIpad guard is correct and minimal: desktop already satisfied both conditions so its behaviour is unchanged, and mobile/iPad now render the volume slider for lines and PGM. The program-output-line guard is preserved. The added regression test exercises all three branches (regular line, PGM non-program user, PGM program user) and would fail if the device gate were reintroduced. No scope creep. CI (lint/prettier/builds/tests) green.