Bracket what-if mode: pick winners, watch the tree fill

A wand toggle turns the bracket interactive: tap a side to send that
team through; W/L placeholders resolve down the tree (pure, tested
logic in src/lib/whatif.ts). Real results always override picks; picks
that stop making sense after an upstream change are pruned. Each open
pairing shows the model's advance probability (ratings.json fetched
lazily), and once your final resolves, a summary banner names your
champion with the model's chance of exactly that run. EN/DE included.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/lib/i18n/en.ts

@@ -157,6 +157,11 @@ export const en = {
     subtitle: "The knockout rounds — from the Round of 32 to the Final.",
     thirdPlacePlayoff: "Third-place play-off",
     advanceTooltip: "Model: each team's chance to advance",
+    whatIf: "What if?",
+    whatIfHint: "Tap a team to send them through the bracket. Real results stay fixed. Percentages are the model's view of each pairing.",
+    reset: "Reset picks",
+    champion: "Your champion: {team}",
+    pathChance: "Model chance of exactly this run: {pct}",
   },
   teams: {
     title: "Teams",