onboarding+liveactivity+habits: 3 fixes from device feedback (KC-61)

1. Onboarding Habits caption now explicitly mentions the toggles are
   changeable in Settings, not just times/custom reminder.

2. TaskLiveActivity: removed the checkbox-look circle glyph (hourglass
   instead). iOS has no API for an app to block the system's swipe/dismiss
   gesture on a Live Activity -- not attempted. Added a real Stop control via
   a new StopCountdownIntent (LiveActivityIntent, iOS 17+) embedded directly
   in the card (lock screen '×' + Dynamic Island 'Stop Countdown' button) --
   the actual iOS-supported equivalent of 'long press for options', since
   Live Activities don't support notification-style long-press menus.

3. Habit notifications gain 'Not Today' (dismiss only, intentional no-op --
   the lifetime-tally philosophy has no concept of a logged miss) and 'Stop
   Tracking' (destructive-styled; flips the habit's master toggle off and
   cancels every pending notification for that type across all its slots).

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

KisaniCalWidgets/StopCountdownIntent.swift

@@ -0,0 +1,26 @@
+import AppIntents
+import ActivityKit
+
+/// Ends a running task Live Activity directly from its Lock Screen / Dynamic
+/// Island card — no need to open the app. Runs in the widget extension
+/// process (that's how LiveActivityIntent works), so it talks to ActivityKit
+/// directly rather than going through LiveActivityManager (app target only).
+@available(iOS 17.0, *)
+struct StopCountdownIntent: LiveActivityIntent {
+    static var title: LocalizedStringResource = "Stop Countdown"
+    static var description = IntentDescription("Ends this task's live countdown.")
+
+    @Parameter(title: "Task ID")
+    var taskID: String
+
+    init() {}
+    init(taskID: String) { self.taskID = taskID }
+
+    func perform() async throws -> some IntentResult {
+        for activity in Activity<TaskActivityAttributes>.activities
+        where activity.attributes.taskID == taskID {
+            await activity.end(dismissalPolicy: .immediate)
+        }
+        return .result()
+    }
+}