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/Sources/App/AppViewModel.swift

@@ -5,6 +5,7 @@ import Observation
 @Observable
 final class AppViewModel {
     var selectedMailbox: Mailbox = .inbox
+    var selectedLane: Lane?
     var selectedThreadID: MailThread.ID?
     var focusedMessageRouteID: String?
     var searchText = ""
@@ -44,6 +45,10 @@ final class AppViewModel {
             .filter { thread in
                 selectedMailbox == .starred ? thread.isStarred : thread.mailbox == selectedMailbox
             }
+            .filter { thread in
+                guard let lane = selectedLane else { return true }
+                return thread.lane == lane
+            }
             .filter { thread in
                 !showUnreadOnly || thread.isUnread
             }
@@ -51,6 +56,27 @@ final class AppViewModel {
             .sorted { $0.lastUpdated > $1.lastUpdated }
     }
 
+    func laneUnreadCount(_ lane: Lane) -> Int {
+        threads.filter { thread in
+            let inCurrentMailbox = selectedMailbox == .starred
+                ? thread.isStarred
+                : thread.mailbox == selectedMailbox
+            return inCurrentMailbox && thread.lane == lane && thread.isUnread
+        }.count
+    }
+
+    func laneThreadCount(_ lane: Lane) -> Int {
+        threads.filter { thread in
+            thread.mailbox == .inbox && thread.lane == lane
+        }.count
+    }
+
+    func selectLane(_ lane: Lane?) {
+        selectedLane = lane
+        clearThreadSelection()
+        mailboxNavigationToken = UUID()
+    }
+
     var totalUnreadCount: Int {
         threads.filter(\.isUnread).count
     }

swift/Sources/App/SocialIOApp.swift

@@ -7,7 +7,7 @@ struct SocialIOApp: App {
     var body: some Scene {
         WindowGroup {
             MailRootView(model: model)
-                .tint(MailTheme.accent)
+                .tint(SIO.tint)
                 .onOpenURL { url in
                     model.apply(url: url)
                 }