Advanced skills can no longer be unlocked through proficiency picks

Advanced skills must be unlocked with the pool cost. Only a profession's
own proficiency still unlocks its skill for free. Proficiency picks need
an unlocked skill, locking a skill returns its picks, old characters that
used picks to unlock a skill get it unlocked automatically.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HJhXuLD2Z6hv3Wqeq9UL9K

backend/app/derive.py

@@ -37,7 +37,7 @@ def compute(ch: dict, rules: dict) -> dict:
         pips = prof_pip + max(0, free_pips)
         prof = [0, cr["proficiency_bonus"], cr["expertise_bonus"]][pips]
         unlocked = s["id"] in (ch.get("unlocked") or [])
-        usable = s["type"] == "basic" or unlocked or prof > 0  # advanced skills must be unlocked
+        usable = s["type"] == "basic" or unlocked or prof_pip > 0  # advanced skills must be unlocked (a profession proficiency does it for free)
         best = max(attrs[a] for a in s["attrs"])  # with several attributes the higher one counts
         skills.append({**s, "points": pts, "prof": prof, "pips": pips, "usable": usable, "unlocked": unlocked,
                        "total": best + pts + prof})