Align Mail UI with social.io design handoff

Rewrite the Mail feature to match the Apple-native look from the
handoff spec: lane-split inbox, AI summary card, clean ThreadRow,
Cc/From + format toolbar in Compose. Drop the gradient hero
surfaces and blurred canvas backgrounds the spec calls out as
anti-patterns, and introduce a token-backed design layer so the
lane palette and SIO tint live in the asset catalog.

- Add Assets.xcassets with SIOTint, LaneFeed, LanePaper, LanePeople
  (light + dark variants).
- Add Sources/Core/Design/SIODesign.swift: SIO tokens, Lane enum,
  LaneChip, AvatarView, AISummaryCard, KeyboardHint, button styles,
  and a glass-chrome helper with iOS 26 / material fallback.
- Extend MailThread with lane + summary; custom Codable keeps old
  payloads decodable. Seed mock threads with sensible lanes and
  hand-write summaries on launch-copy, investor-update, roadmap-notes.
- Add lane filtering to AppViewModel (selectedLane, selectLane,
  laneUnreadCount, laneThreadCount).
- Rewrite MailRootView end to end: sidebar with Inbox/lane rows,
  lane filter strip, Apple-native ThreadRow (avatar, unread dot,
  lane chip, summary chip), ThreadReadingView with AI summary +
  floating reply pill, ComposeView with To/Cc/From/Subject and a
  format toolbar.
- Wire Assets.xcassets + SIODesign.swift into project.pbxproj.

Accessibility identifiers preserved byte-identical; new ones
(mailbox.lane.*, lane.chip.*) added only where new.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

swift/SocialIO.xcodeproj/project.pbxproj

@@ -16,6 +16,8 @@
 		A10000000000000000000007 /* AppControlService.swift in Sources */ = {isa = PBXBuildFile; fileRef = A20000000000000000000008 /* AppControlService.swift */; };
 		A10000000000000000000008 /* AppNavigationCommandTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = A20000000000000000000009 /* AppNavigationCommandTests.swift */; };
 		A10000000000000000000009 /* AppViewModelTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = A2000000000000000000000A /* AppViewModelTests.swift */; };
+		A10000000000000000000010 /* SIODesign.swift in Sources */ = {isa = PBXBuildFile; fileRef = A20000000000000000000020 /* SIODesign.swift */; };
+		A10000000000000000000011 /* Assets.xcassets in Resources */ = {isa = PBXBuildFile; fileRef = A20000000000000000000021 /* Assets.xcassets */; };
 /* End PBXBuildFile section */
 
 /* Begin PBXContainerItemProxy section */
@@ -40,6 +42,8 @@
 		A20000000000000000000009 /* AppNavigationCommandTests.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = AppNavigationCommandTests.swift; sourceTree = "<group>"; };
 		A2000000000000000000000A /* AppViewModelTests.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = AppViewModelTests.swift; sourceTree = "<group>"; };
 		A2000000000000000000000B /* SocialIOTests.xctest */ = {isa = PBXFileReference; explicitFileType = wrapper.cfbundle; includeInIndex = 0; path = SocialIOTests.xctest; sourceTree = BUILT_PRODUCTS_DIR; };
+		A20000000000000000000020 /* SIODesign.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = SIODesign.swift; sourceTree = "<group>"; };
+		A20000000000000000000021 /* Assets.xcassets */ = {isa = PBXFileReference; lastKnownFileType = folder.assetcatalog; path = Assets.xcassets; sourceTree = "<group>"; };
 /* End PBXFileReference section */
 
 /* Begin PBXFrameworksBuildPhase section */
@@ -73,6 +77,7 @@
 			children = (
 				A40000000000000000000003 /* Sources */,
 				A4000000000000000000000B /* Tests */,
+				A20000000000000000000021 /* Assets.xcassets */,
 			);
 			name = SocialIO;
 			sourceTree = "<group>";
@@ -103,10 +108,19 @@
 			children = (
 				A40000000000000000000006 /* Models */,
 				A40000000000000000000007 /* Services */,
+				A40000000000000000000020 /* Design */,
 			);
 			path = Core;
 			sourceTree = "<group>";
 		};
+		A40000000000000000000020 /* Design */ = {
+			isa = PBXGroup;
+			children = (
+				A20000000000000000000020 /* SIODesign.swift */,
+			);
+			path = Design;
+			sourceTree = "<group>";
+		};
 		A40000000000000000000006 /* Models */ = {
 			isa = PBXGroup;
 			children = (
@@ -238,6 +252,7 @@
 			isa = PBXResourcesBuildPhase;
 			buildActionMask = 2147483647;
 			files = (
+				A10000000000000000000011 /* Assets.xcassets in Resources */,
 			);
 			runOnlyForDeploymentPostprocessing = 0;
 		};
@@ -262,6 +277,7 @@
 				A10000000000000000000001 /* SocialIOApp.swift in Sources */,
 				A10000000000000000000006 /* AppNavigationCommand.swift in Sources */,
 				A10000000000000000000007 /* AppControlService.swift in Sources */,
+				A10000000000000000000010 /* SIODesign.swift in Sources */,
 			);
 			runOnlyForDeploymentPostprocessing = 0;
 		};