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>

static/v3/profile.js

@@ -375,7 +375,7 @@
             // Step 3 — Achievements wall opt-in (first-run only; default OFF).
             '<div id="v3-ob-step3" class="hidden">' +
             '<label class="block text-xs uppercase tracking-wider text-fb-textDim mb-2">Feats of Power</label>' +
-            '<p class="text-sm text-fb-textDim mb-3">As you practise you’ll earn rare <span class="text-fb-text">Feats of Power</span> — silly, bombastic activity trophies. Want to show them off on the public <span class="text-fb-text">Feats wall</span>?</p>' +
+            '<p class="text-sm text-fb-textDim mb-3">As you practice you’ll earn rare <span class="text-fb-text">Feats of Power</span> — silly, bombastic activity trophies. Want to show them off on the public <span class="text-fb-text">Feats wall</span>?</p>' +
             '<label class="flex items-start gap-3 cursor-pointer rounded-lg border border-fb-border/50 bg-fb-bg/40 p-3">' +
             '<input type="checkbox" id="v3-ob-optin" class="mt-1 h-4 w-4 rounded border-gray-600 bg-gray-800 text-fb-primary focus:ring-fb-primary">' +
             '<span class="text-sm text-fb-text">Share my Feats on the wall' +