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/Core/Models/MailModels.swift

@@ -80,6 +80,8 @@ struct MailThread: Identifiable, Hashable, Codable {
     var isUnread: Bool
     var isStarred: Bool
     var tags: [String]
+    var lane: Lane
+    var summary: [String]?
 
     init(
         id: UUID = UUID(),
@@ -90,7 +92,9 @@ struct MailThread: Identifiable, Hashable, Codable {
         messages: [MailMessage],
         isUnread: Bool,
         isStarred: Bool,
-        tags: [String] = []
+        tags: [String] = [],
+        lane: Lane = .people,
+        summary: [String]? = nil
     ) {
         self.id = id
         self.routeID = routeID
@@ -101,6 +105,29 @@ struct MailThread: Identifiable, Hashable, Codable {
         self.isUnread = isUnread
         self.isStarred = isStarred
         self.tags = tags
+        self.lane = lane
+        self.summary = summary
+    }
+
+    enum CodingKeys: String, CodingKey {
+        case id, routeID, mailbox, subject, participants, messages
+        case isUnread, isStarred, tags, lane, summary
+    }
+
+    init(from decoder: Decoder) throws {
+        let c = try decoder.container(keyedBy: CodingKeys.self)
+        self.id = try c.decode(UUID.self, forKey: .id)
+        self.routeID = try c.decode(String.self, forKey: .routeID)
+        self.mailbox = try c.decode(Mailbox.self, forKey: .mailbox)
+        self.subject = try c.decode(String.self, forKey: .subject)
+        self.participants = try c.decode([MailPerson].self, forKey: .participants)
+        let rawMessages = try c.decode([MailMessage].self, forKey: .messages)
+        self.messages = rawMessages.sorted { $0.sentAt < $1.sentAt }
+        self.isUnread = try c.decode(Bool.self, forKey: .isUnread)
+        self.isStarred = try c.decode(Bool.self, forKey: .isStarred)
+        self.tags = try c.decodeIfPresent([String].self, forKey: .tags) ?? []
+        self.lane = try c.decodeIfPresent(Lane.self, forKey: .lane) ?? .people
+        self.summary = try c.decodeIfPresent([String].self, forKey: .summary)
     }
 
     var latestMessage: MailMessage? {