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/Services/MockMailService.swift

@@ -82,7 +82,13 @@ struct MockMailService: MailServicing {
                 ],
                 isUnread: true,
                 isStarred: true,
-                tags: ["Design", "Launch"]
+                tags: ["Design", "Launch"],
+                lane: .people,
+                summary: [
+                    "Tanya tightened the onboarding copy and softened the empty-state tone.",
+                    "Phil signed off with a note to keep playful language on iPhone.",
+                    "Desktop first-run copy still needs a lighter pass before handoff."
+                ]
             ),
             MailThread(
                 routeID: "daily-sync-status",
@@ -100,7 +106,8 @@ struct MockMailService: MailServicing {
                 ],
                 isUnread: false,
                 isStarred: false,
-                tags: ["System"]
+                tags: ["System"],
+                lane: .feed
             ),
             MailThread(
                 routeID: "investor-update",
@@ -118,7 +125,13 @@ struct MockMailService: MailServicing {
                 ],
                 isUnread: true,
                 isStarred: false,
-                tags: ["External"]
+                tags: ["External"],
+                lane: .paper,
+                summary: [
+                    "Mina wants a concise product update before Friday.",
+                    "She's specifically asking for screenshots of the new mail experience.",
+                    "Interested in how we differentiate from commodity inboxes."
+                ]
             ),
             MailThread(
                 routeID: "search-ranking-polish",
@@ -143,7 +156,8 @@ struct MockMailService: MailServicing {
                 ],
                 isUnread: false,
                 isStarred: false,
-                tags: ["Search"]
+                tags: ["Search"],
+                lane: .people
             ),
             MailThread(
                 routeID: "welcome-to-socialio",
@@ -162,7 +176,8 @@ struct MockMailService: MailServicing {
                 ],
                 isUnread: false,
                 isStarred: false,
-                tags: ["Draft"]
+                tags: ["Draft"],
+                lane: .people
             ),
             MailThread(
                 routeID: "roadmap-notes",
@@ -180,7 +195,13 @@ struct MockMailService: MailServicing {
                 ],
                 isUnread: false,
                 isStarred: true,
-                tags: ["Product"]
+                tags: ["Product"],
+                lane: .paper,
+                summary: [
+                    "Three roadmap themes: faster triage, identity-rich threads, calmer notifications.",
+                    "Raw notes were cleaned up and archived by Nora.",
+                    "Owner split still needs to be reconciled with eng capacity."
+                ]
             )
         ]
     }