feat(web): add i18n, light/dark theme switching, and version constant

- Add i18n (English + 简体中文) with a locale store, $t() magic, and a language toggle (persisted)
- Add light/dark/system theme switching via a theme store and [data-theme] token overrides
- Set the theme before paint to avoid a flash of the wrong theme
- Add frontend version constant web/version.js (window.YDROPBX_VERSION) shown in the app footer
- Update AGENTS.md version-control convention to mention web/version.js
- Bump version to 0.1.1 across frontend, webui changelog, and backend constant

web/login.html

@@ -7,16 +7,44 @@
     <link rel="stylesheet" href="/styles/tokens.css">
     <link rel="stylesheet" href="/styles/base.css">
     <link rel="stylesheet" href="/styles/components.css">
+
+    <script>
+      (function () {
+        try {
+          var t = localStorage.getItem('yd.theme') || 'system';
+          var dark = t === 'dark' || (t === 'system' && window.matchMedia('(prefers-color-scheme: dark)').matches);
+          document.documentElement.setAttribute('data-theme', dark ? 'dark' : 'light');
+        } catch (e) {}
+      })();
+    </script>
+
+    <script defer src="/components/icons.js"></script>
+    <script defer src="/components/i18n.js"></script>
+    <script defer src="/components/theme.js"></script>
+    <script defer src="/alpine.min.js"></script>
 </head>
-<body>
+<body x-data>
+    <div class="page-topbar">
+        <button class="btn btn-icon" type="button"
+                @click="$store.locale.toggle()"
+                :title="$t('lang.label')" :aria-label="$t('lang.label')">
+            <span class="icon" x-html="Icons.globe"></span>
+        </button>
+        <button class="btn btn-icon" type="button"
+                @click="$store.theme.cycle()"
+                :title="$t('theme.' + $store.theme.mode)" :aria-label="$t('theme.' + $store.theme.mode)">
+            <span class="icon" x-html="Icons[$store.theme.icon()]"></span>
+        </button>
+    </div>
+
     <div class="login-wrap">
         <div class="login-card">
             <h1>y<span>Dropbox</span></h1>
-            <p>Enter your access token to continue</p>
+            <p x-text="$t('login.subtitle')"></p>
             <form id="loginForm">
-                <input type="password" id="token" placeholder="Access Token" required autocomplete="current-password">
-                <div class="login-error" id="loginError">Invalid token. Please try again.</div>
-                <button type="submit" class="btn btn-primary">Sign In</button>
+                <input type="password" id="token" :placeholder="$t('login.token')" required autocomplete="current-password">
+                <div class="login-error" id="loginError" x-text="$t('login.error')"></div>
+                <button type="submit" class="btn btn-primary" x-text="$t('login.submit')"></button>
             </form>
         </div>
     </div>