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/styles/tokens.css

@@ -21,6 +21,13 @@
 
   --color-success: #10b981;
 
+  /* Structural tokens */
+  --color-divider: #f3f4f6;
+  --color-toast-bg: #1a1d23;
+  --color-toast-fg: #ffffff;
+
+  color-scheme: light;
+
   /* ===== Spacing (4px base) ===== */
   --space-1: 0.25rem;
   --space-2: 0.5rem;
@@ -61,3 +68,31 @@
   --z-toast: 1000;
   --z-modal: 1100;
 }
+
+/* ===== Dark theme ===== */
+[data-theme="dark"] {
+  --color-bg: #0f1115;
+  --color-bg-gradient: radial-gradient(1200px 600px at 50% -10%, #1a2030 0%, var(--color-bg) 55%);
+  --color-surface: #171a21;
+  --color-surface-hover: #1e222b;
+  --color-border: #262b35;
+  --color-border-strong: #323844;
+
+  --color-text: #e6e8ec;
+  --color-text-secondary: #9aa1ad;
+  --color-text-tertiary: #6b7280;
+
+  --color-primary: #3b82f6;
+  --color-primary-hover: #2f6fe0;
+  --color-primary-soft: #16233f;
+
+  --color-danger: #f87171;
+  --color-danger-hover: #ef4444;
+  --color-danger-soft: #2a1717;
+
+  --color-divider: #23262f;
+  --color-toast-bg: #e6e8ec;
+  --color-toast-fg: #1a1d23;
+
+  color-scheme: dark;
+}