feat: public read-only share links with permissions — closes #79

Share links:
- Generate a public link in the trip share modal
- Choose what to share: Map & Plan, Bookings, Packing, Budget, Chat
- Permissions enforced server-side
- Delete link to revoke access instantly

Shared trip page (/shared/:token):
- Read-only view with TREK logo, cover image, trip details
- Tabbed navigation with Lucide icons (responsive on mobile)
- Interactive map with auto-fit bounds per day
- Day plan, Bookings, Packing, Budget, Chat views
- Language picker, TREK branding footer

Technical:
- share_tokens DB table with per-field permissions
- Public GET /shared/:token endpoint (no auth)
- Two-column share modal (max-w-5xl)

server/src/db/migrations.ts

@@ -356,6 +356,29 @@ function runMigrations(db: Database.Database): void {
       try { db.exec('ALTER TABLE notification_preferences ADD COLUMN notify_collab_message INTEGER DEFAULT 1'); } catch {}
       try { db.exec('ALTER TABLE notification_preferences ADD COLUMN notify_packing_tagged INTEGER DEFAULT 1'); } catch {}
     },
+    () => {
+      // Public share links for read-only trip access
+      db.exec(`CREATE TABLE IF NOT EXISTS share_tokens (
+        id INTEGER PRIMARY KEY AUTOINCREMENT,
+        trip_id INTEGER NOT NULL REFERENCES trips(id) ON DELETE CASCADE,
+        token TEXT NOT NULL UNIQUE,
+        created_by INTEGER NOT NULL REFERENCES users(id),
+        share_map INTEGER DEFAULT 1,
+        share_bookings INTEGER DEFAULT 1,
+        share_packing INTEGER DEFAULT 0,
+        share_budget INTEGER DEFAULT 0,
+        share_collab INTEGER DEFAULT 0,
+        created_at DATETIME DEFAULT CURRENT_TIMESTAMP
+      )`);
+    },
+    () => {
+      // Add permission columns to share_tokens
+      try { db.exec('ALTER TABLE share_tokens ADD COLUMN share_map INTEGER DEFAULT 1'); } catch {}
+      try { db.exec('ALTER TABLE share_tokens ADD COLUMN share_bookings INTEGER DEFAULT 1'); } catch {}
+      try { db.exec('ALTER TABLE share_tokens ADD COLUMN share_packing INTEGER DEFAULT 0'); } catch {}
+      try { db.exec('ALTER TABLE share_tokens ADD COLUMN share_budget INTEGER DEFAULT 0'); } catch {}
+      try { db.exec('ALTER TABLE share_tokens ADD COLUMN share_collab INTEGER DEFAULT 0'); } catch {}
+    },
   ];
 
   if (currentVersion < migrations.length) {