Deliver localized auth app with verification and go_router

Integrate themed login/signup, EN/RU localization, shared dummy providers, verification recovery screens and named URL routes. Store frontend guidelines, workflow skills, validation and delivery documentation. Related to #2, #3, #5 and #6.

frontend/app/web/favicon.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 64 64"><rect width="64" height="64" rx="16" fill="#14221d"/><path d="M19 45 45 19M20 19h25v25" fill="none" stroke="#c4f9ce" stroke-width="7" stroke-linecap="round" stroke-linejoin="round"/></svg>

frontend/app/web/index.html

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+<!DOCTYPE html>
+<html lang="en">
+<head>
+  <!--
+    If you are serving your web app in a path other than the root, change the
+    href value below to reflect the base path you are serving from.
+
+    The path provided below has to start and end with a slash "/" in order for
+    it to work correctly.
+
+    For more details:
+    * https://developer.mozilla.org/en-US/docs/Web/HTML/Element/base
+
+    This is a placeholder for base href that will be replaced by the value of
+    the `--base-href` argument provided to `flutter build`.
+  -->
+  <base href="$FLUTTER_BASE_HREF">
+
+  <meta charset="UTF-8">
+  <meta name="robots" content="noindex,nofollow">
+  <meta content="IE=Edge" http-equiv="X-UA-Compatible">
+  <meta name="description" content="SXCHNGE account preview.">
+
+  <!-- iOS meta tags & icons -->
+  <meta name="mobile-web-app-capable" content="yes">
+  <meta name="apple-mobile-web-app-status-bar-style" content="black">
+  <meta name="apple-mobile-web-app-title" content="SXCHNGE">
+
+
+  <!-- Favicon -->
+  <link rel="icon" type="image/svg+xml" href="favicon.svg"/>
+
+  <title>SXCHNGE</title>
+  <link rel="manifest" href="manifest.json">
+</head>
+<body>
+  <!--
+    You can customize the "flutter_bootstrap.js" script.
+    This is useful to provide a custom configuration to the Flutter loader
+    or to give the user feedback during the initialization process.
+
+    For more details:
+    * https://docs.flutter.dev/platform-integration/web/initialization
+  -->
+  <script src="flutter_bootstrap.js" async></script>
+</body>
+</html>

frontend/app/web/manifest.json

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+{
+  "name": "SXCHNGE",
+  "short_name": "SXCHNGE",
+  "start_url": ".",
+  "display": "standalone",
+  "background_color": "#f9faf6",
+  "theme_color": "#14221d",
+  "description": "SXCHNGE account preview.",
+  "orientation": "any",
+  "prefer_related_applications": false,
+  "icons": [
+    {
+      "src": "favicon.svg",
+      "sizes": "any",
+      "type": "image/svg+xml",
+      "purpose": "any"
+    }
+  ]
+}