Matrix: backfill legacy task priorities so birthdays place correctly (KC-21)

The Priority defaults only fired at creation, so tasks saved before Priority
existed (e.g. birthdays) all had `.none` and fell to the Matrix bottom row
instead of Q2. Added a one-time, flag-guarded migration on load that applies
the KisaniCal category defaults (birthday/domain/annual/exam/subscription →
High, workout → Medium) to existing `.none`-priority tasks.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

KisaniCal/Models/TaskItem.swift

@@ -152,6 +152,22 @@ class TaskViewModel: ObservableObject {
         } else {
             tasks = []
         }
+        migrateCategoryPrioritiesIfNeeded()
+    }
+
+    /// One-time backfill: tasks created before Priority existed all have `.none`.
+    /// Apply the KisaniCal category defaults (birthdays → High, etc.) so the Matrix
+    /// places legacy birthdays/renewals in the top row like newly-created ones.
+    private func migrateCategoryPrioritiesIfNeeded() {
+        let flag = "kisani.migration.categoryPriority.v1"
+        guard !UserDefaults.kisani.bool(forKey: flag) else { return }
+        var changed = false
+        for idx in tasks.indices where tasks[idx].priority == .none {
+            let def = defaultPriority(category: tasks[idx].category, title: tasks[idx].title)
+            if def != .none { tasks[idx].priority = def; changed = true }
+        }
+        UserDefaults.kisani.set(true, forKey: flag)
+        if changed { save() }
     }
 
     func reload() {