feat(design): 主色改靛蓝 #4F46E5，Web 与安卓及 Logo 同步

- primary 换成 Indigo 阶（600 即 #4F46E5），中性色转 Slate 轴、纸面更冷更白
- 实心主色面（主按钮、工作区选中）用 600，链接/浅底文字仍用 700 保对比度
- 信息色 sky 调深到 #16697C，与靛蓝同屏能分清
- Logo / favicon / PWA 图标 / 安卓自适应图标统一为靛蓝底 + 白线 + 淡紫点，tools/make-icons.py 重新生成
- 文档与 e2e 断言同步（主题色 #4f46e5）

android/app/src/main/java/com/dadao/task/ui/KnowledgeScreen.kt

@@ -329,7 +329,7 @@ private fun KnowledgeBody(text: String) {
                 setTextIsSelectable(true)
                 textSize = 14f
                 setLineSpacing(0f, 1.5f)
-                setTextColor(android.graphics.Color.parseColor("#262D39"))
+                setTextColor(android.graphics.Color.parseColor("#1E293B"))
                 markwon.setMarkdown(this, text)
             }
         },

android/app/src/main/java/com/dadao/task/ui/RichTextEditorScreen.kt

@@ -350,7 +350,7 @@ private fun PreviewBody(
                         textSize = 15f
                         setPadding(0, 0, 0, 0)
                         setLineSpacing(0f, 1.5f)
-                        setTextColor(android.graphics.Color.parseColor("#4E5867"))
+                        setTextColor(android.graphics.Color.parseColor("#475569"))
                         markwon.setMarkdown(this, text)
                     }
                 },

android/app/src/main/java/com/dadao/task/ui/Theme.kt

@@ -13,22 +13,22 @@ import androidx.compose.ui.unit.dp
 import androidx.compose.ui.unit.sp
 
 /**
- * 与 Web 端 tailwind.config.js 同一套色板：暖白纸面 + 景泰蓝主色 + 陶土橙强调。
+ * 与 Web 端 tailwind.config.js 同一套色板：冷白纸面 + 靛蓝主色（#4F46E5）+ 陶土橙强调。
  * 两端看同一张截图时，颜色应该能对上，所以这里的值不要单独调。
  */
-val Paper = Color(0xFFF9FAFC)
-val PaperSunken = Color(0xFFF3F5F9)
-val Line = Color(0xFFE2E6EC)
-val LineStrong = Color(0xFFD3D9E1)
+val Paper = Color(0xFFFAFAFC)
+val PaperSunken = Color(0xFFF4F5F9)
+val Line = Color(0xFFE3E7EF)
+val LineStrong = Color(0xFFD5DAE4)
 
-val Ink = Color(0xFF262D39)
-val InkSoft = Color(0xFF4E5867)
-val InkMuted = Color(0xFF5F6B7C)
+val Ink = Color(0xFF1E293B)
+val InkSoft = Color(0xFF475569)
+val InkMuted = Color(0xFF64748B)
 
-val Primary = Color(0xFF274B73)
-val PrimaryDeep = Color(0xFF1F3B5B)
-val PrimarySoft = Color(0xFFDCE9F7)
-val PrimaryTint = Color(0xFFB9D3EC)
+val Primary = Color(0xFF4F46E5)
+val PrimaryDeep = Color(0xFF4338CA)
+val PrimarySoft = Color(0xFFE0E7FF)
+val PrimaryTint = Color(0xFFC7D2FE)
 
 val Clay = Color(0xFFC2703D)
 val ClaySoft = Color(0xFFFBEADF)
@@ -39,7 +39,7 @@ val Wheat = Color(0xFFA97C25)
 val WheatSoft = Color(0xFFF8EED7)
 val Rust = Color(0xFFB04A32)
 val RustSoft = Color(0xFFFBE5DF)
-val Sky = Color(0xFF1E7F94)
+val Sky = Color(0xFF16697C)
 val SkySoft = Color(0xFFE6F4F6)
 
 private val colors = lightColorScheme(