feat(mobile): advance Paca.Mobile to production excellence with rich design assets and core systems

- Generate high-resolution glassmorphism app icon and splash screen assets
- Implement MobileIconSystem with semantic SF Symbols/Material glyph mappings
- Add OLEDBlack theme and contrast tokens to MobileThemeSystem
- Implement VideoSnifferParser.NegotiateBestQuality for multi-stream quality resolution
- Enhance DiscoverPage, FavoritesPage, and SyncSettingsPage with production controls
- Add Gate 103-114 test suites with 100% test pass rate

tests/Paca.Tests/Mobile/MobileMediaPlaybackGateTests.cs

@@ -0,0 +1,107 @@
+using System;
+using System.Collections.Generic;
+using Paca.Mobile.Core;
+using Xunit;
+
+namespace Paca.Tests.Mobile;
+
+/// <summary>
+/// 모바일 프로덕션 미디어 플레이어 제스처, 자막, 배속, 스토리지 및 공유 게이트 (Gate 103 ~ Gate 108)
+/// </summary>
+public class MobileMediaPlaybackGateTests
+{
+    [Fact]
+    public void Gate103_MobilePlayerGestureEngine_Audit()
+    {
+        var gesture = new MobilePlayerGestureEngine();
+
+        // 1. 수평 스와이프 -> Seek
+        var seekResult = gesture.OnHorizontalPan(deltaX: 100, currentPositionSec: 30, durationSec: 300);
+        Assert.True(seekResult.NewPositionSec > 30);
+
+        // 2. 좌측 수직 스와이프 -> 밝기 조절
+        var brightness = gesture.OnVerticalPan(isLeftScreen: true, deltaY: -50, currentBrightness: 0.5);
+        Assert.True(brightness > 0.5);
+
+        // 3. 우측 수직 스와이프 -> 볼륨 조절
+        var volume = gesture.OnVerticalPan(isLeftScreen: false, deltaY: -50, currentVolume: 0.5);
+        Assert.True(volume > 0.5);
+
+        // 4. 더블 탭 -> 토글
+        Assert.True(gesture.OnDoubleTap(isPlaying: false));
+        Assert.False(gesture.OnDoubleTap(isPlaying: true));
+    }
+
+    [Fact]
+    public void Gate104_MobilePlaybackSpeedController_Audit()
+    {
+        var controller = new MobilePlaybackSpeedController();
+        Assert.Equal(1.0, controller.CurrentSpeed);
+
+        controller.SetSpeed(1.5);
+        Assert.Equal(1.5, controller.CurrentSpeed);
+
+        controller.SetSpeed(3.0);
+        Assert.Equal(3.0, controller.CurrentSpeed);
+
+        // 음높이(Pitch) 보정 활성화
+        Assert.True(controller.PitchCorrectionEnabled);
+    }
+
+    [Fact]
+    public void Gate105_MobileSubtitleRenderer_Audit()
+    {
+        var renderer = new MobileSubtitleRenderer();
+        var vttSample = "WEBVTT\n\n00:00:01.000 --> 00:00:04.000\n안녕하세요, Paca 비디오 다운로더입니다.\n";
+        renderer.LoadSubtitles(vttSample);
+
+        Assert.Single(renderer.Cues);
+        var activeCue = renderer.GetActiveCue(TimeSpan.FromSeconds(2));
+        Assert.NotNull(activeCue);
+        Assert.Equal("안녕하세요, Paca 비디오 다운로더입니다.", activeCue.Text);
+
+        // 자막 스타일
+        renderer.FontSizeMode = SubtitleFontSize.Large;
+        Assert.Equal(SubtitleFontSize.Large, renderer.FontSizeMode);
+    }
+
+    [Fact]
+    public void Gate106_MobilePipController_Audit()
+    {
+        var pip = new MobilePipController();
+        Assert.False(pip.IsPipActive);
+        Assert.True(pip.BackgroundAudioEnabled);
+
+        pip.EnterPipMode();
+        Assert.True(pip.IsPipActive);
+
+        pip.ExitPipMode();
+        Assert.False(pip.IsPipActive);
+    }
+
+    [Fact]
+    public void Gate107_MobileStoragePolicy_Audit()
+    {
+        var policy = new MobileStoragePolicy();
+        // 90% 임계값
+        Assert.Equal(90.0, policy.WarningThresholdPercent);
+
+        // 95% 사용 시 경고 발생 검증
+        var warning = policy.CheckStorageUsage(totalBytes: 100_000_000, usedBytes: 95_000_000);
+        Assert.True(warning.IsWarningTriggered);
+        Assert.Equal(95.0, warning.UsagePercent);
+    }
+
+    [Fact]
+    public void Gate108_MobileMediaExporter_Audit()
+    {
+        var exporter = new MobileMediaExporter();
+        var exportResult = exporter.PrepareExport("sample_video.mp4", isAudioOnly: false);
+        Assert.Equal("video/mp4", exportResult.MimeType);
+        Assert.Contains("Movies", exportResult.DestinationFolder);
+
+        var audioResult = exporter.PrepareExport("sample_audio.mp3", isAudioOnly: true);
+        Assert.Equal("audio/mpeg", audioResult.MimeType);
+        Assert.Contains("Music", audioResult.DestinationFolder);
+    }
+}