fix: "practise" -> "practice" (American spelling, matches the rest of the UI)

Three spots used British "practise" while the rest of the app uses
American "practice" (Keep practicing, Virtuoso - Practice, etc.):
- static/v3/profile.js — first-run onboarding "Feats of Power" step
- plugins/achievements/feats.json — The Witching Hour feat description
- plugins/achievements/engine.py — code comment

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Signed-off-by: topkoa <topkoa@gmail.com>

plugins/achievements/engine.py

@@ -109,7 +109,7 @@ def apply_activity(counters, delta):
 def consecutive_run_length(dates):
     """Longest run of consecutive calendar dates in ``dates`` (ISO 'YYYY-MM-DD').
 
-    Used by the `secret_witching` Feat (practise in the 2–5am window on N
+    Used by the `secret_witching` Feat (practice in the 2–5am window on N
     consecutive nights). Pure date arithmetic so it's unit-testable; routes.py
     feeds it the distinct night-dates recorded in `comp_ledger`.
     """

plugins/achievements/feats.json

@@ -76,7 +76,7 @@
     {
       "id": "secret_witching",
       "title": "The Witching Hour",
-      "description": "Practise in the dead of night, seven nights running.",
+      "description": "Practice in the dead of night, seven nights running.",
       "category": "global",
       "sourceId": "achievements",
       "secret": true,