feat(mobile): Learn tab v2, forecast details sheet, and UI polish

src/mobile/lib/src/app/dashboard/models/forecast_guidance.dart

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+import 'package:airqo/src/app/dashboard/models/forecast_response.dart';
+
+/// API-sourced guidance for the forecast modal guidance panel.
+class ForecastGuidance {
+  final String? message;
+  final String? trendMessage;
+
+  const ForecastGuidance({
+    this.message,
+    this.trendMessage,
+  });
+
+  bool get hasContent => _nonEmpty(message) || _nonEmpty(trendMessage);
+
+  static bool _nonEmpty(String? value) =>
+      value != null && value.trim().isNotEmpty;
+}
+
+ForecastGuidance guidanceFromForecast(Forecast forecast) {
+  return ForecastGuidance(
+    message: _trimOrNull(forecast.aqiLabel),
+    trendMessage: _trimOrNull(forecast.trendMessage),
+  );
+}
+
+ForecastGuidance guidanceFromHourlyEntry(HourlyForecastEntry entry) {
+  return ForecastGuidance(
+    message: _trimOrNull(entry.aqiLabel),
+    trendMessage: _trimOrNull(entry.trendMessage),
+  );
+}
+
+String? _trimOrNull(String? value) {
+  if (value == null) return null;
+  final trimmed = value.trim();
+  return trimmed.isEmpty ? null : trimmed;
+}
+
+/// Normalized reading fields for the shared forecast detail card.
+class ForecastReadingSnapshot {
+  final double pm25;
+  final String aqiCategory;
+  final String aqiColor;
+  final double? forecastConfidence;
+  final ForecastMet? met;
+
+  const ForecastReadingSnapshot({
+    required this.pm25,
+    required this.aqiCategory,
+    required this.aqiColor,
+    this.forecastConfidence,
+    this.met,
+  });
+
+  factory ForecastReadingSnapshot.fromDaily(Forecast forecast) {
+    return ForecastReadingSnapshot(
+      pm25: forecast.pm25,
+      aqiCategory: forecast.aqiCategory,
+      aqiColor: forecast.aqiColor,
+      forecastConfidence: forecast.forecastConfidence,
+      met: forecast.met,
+    );
+  }
+
+  factory ForecastReadingSnapshot.fromHourly(HourlyForecastEntry entry) {
+    return ForecastReadingSnapshot(
+      pm25: entry.pm25Mean,
+      aqiCategory: entry.aqiCategory,
+      aqiColor: entry.aqiColor,
+      forecastConfidence: entry.forecastConfidence,
+      met: entry.met,
+    );
+  }
+}
+
+List<HourlyForecastEntry> hourlyEntriesForDate(
+  HourlyForecastResponse? response,
+  DateTime date,
+) {
+  if (response == null) return [];
+  final dateStr = _fmtDate(date.toLocal());
+  return response.forecasts
+      .where((e) => _fmtDate(e.time.toLocal()) == dateStr)
+      .toList();
+}
+
+String _fmtDate(DateTime dt) {
+  return '${dt.year.toString().padLeft(4, '0')}-'
+      '${dt.month.toString().padLeft(2, '0')}-'
+      '${dt.day.toString().padLeft(2, '0')}';
+}
+
+int defaultHourlyIndex(List<HourlyForecastEntry> entries, DateTime selectedDay) {
+  if (entries.isEmpty) return 0;
+  final now = DateTime.now();
+  final todayStr = _fmtDate(now);
+  final dayStr = _fmtDate(selectedDay.toLocal());
+  if (dayStr != todayStr) return 0;
+
+  for (var i = 0; i < entries.length; i++) {
+    if (entries[i].time.toLocal().hour == now.hour) return i;
+  }
+  return 0;
+}