tasks: add "every N weeks/months/days/years" recurrence (KC-70)

Recurrence previously only supported 5 fixed rules with no interval.
Adds TaskItem.recurrenceInterval (optional, nil = 1, so every existing
task is unaffected), a stepper in the shared create/edit sheet, quick-add
NLP for phrases like "every 2 weeks"/"biweekly"/"fortnightly", and
threads the interval through both countdown widgets so their progress
bars size correctly for biweekly/etc tasks too.

Built on KC-69's consolidated RecurrenceRule engine. Verified two things
by actually compiling and running standalone swiftc drivers rather than
just reading: the core interval math (44/44, unchanged from KC-69) and
the new quick-add regex patterns against realistic phrases including
"look into tru housing options every two weeks" (11/11 passing).

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

Shared/RecurrenceRule.swift

@@ -89,4 +89,21 @@ enum RecurrenceRule {
         guard let prev = cal.date(byAdding: comp, value: -step, to: next) else { return nil }
         return (prev, next)
     }
+
+    /// User-facing recurrence text — "Weekly" at interval 1 (unchanged from
+    /// before intervals existed), "Every 2 Weeks" once N > 1. One shared
+    /// formatter so the main app and the create/edit picker UI can't drift.
+    static func displayLabel(label: String, interval: Int = 1) -> String {
+        let n = max(1, interval)
+        guard n > 1, label != "Every Weekday" else { return label }
+        let unit: String
+        switch label {
+        case "Daily":   unit = "Days"
+        case "Weekly":  unit = "Weeks"
+        case "Monthly": unit = "Months"
+        case "Yearly":  unit = "Years"
+        default:        return label
+        }
+        return "Every \(n) \(unit)"
+    }
 }