feat: remote manifest model and content dependencies

Add http, crypto, path_provider deps; introduce RemoteManifest /
ManifestSeason immutable models with fromJson/toJson, schema-version
guard (FormatException on unsupported schema), and fallback for missing
current field. 3/3 TDD tests pass, flutter analyze clean.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

pubspec.lock

@@ -146,7 +146,7 @@ packages:
     source: hosted
     version: "1.15.1"
   crypto:
-    dependency: transitive
+    dependency: "direct main"
     description:
       name: crypto
       sha256: c8ea0233063ba03258fbcf2ca4d6dadfefe14f02fab57702265467a19f27fadf
@@ -246,7 +246,7 @@ packages:
     source: hosted
     version: "2.1.3"
   http:
-    dependency: transitive
+    dependency: "direct main"
     description:
       name: http
       sha256: "87721a4a50b19c7f1d49001e51409bddc46303966ce89a65af4f4e6004896412"
@@ -390,7 +390,7 @@ packages:
     source: hosted
     version: "1.9.1"
   path_provider:
-    dependency: transitive
+    dependency: "direct main"
     description:
       name: path_provider
       sha256: "50c5dd5b6e1aaf6fb3a78b33f6aa3afca52bf903a8a5298f53101fdaee55bbcd"