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/components/store.js

@@ -24,12 +24,12 @@
           this.files.inbox = inbox.files || [];
           this.files.outbox = outbox.files || [];
         } catch (e) {
-          this.showToast('Failed to load files');
+          this.showToast('toast.loadFailed');
         }
       },
 
       showToast(message) {
-        this.toast.message = message;
+        this.toast.message = Alpine.store('locale').t(message);
         this.toast.show = true;
         if (this.toast._timer) clearTimeout(this.toast._timer);
         this.toast._timer = setTimeout(() => {
@@ -45,7 +45,7 @@
           form.append('dir', dir);
           await fetch('/api/upload', { method: 'POST', body: form });
         }
-        this.showToast('File uploaded');
+        this.showToast('toast.uploaded');
         await this.loadFiles();
       },
 
@@ -55,13 +55,13 @@
 
       async deleteFile(id) {
         const confirmed = await Alpine.store('confirm').show({
-          title: 'Delete file',
-          message: 'This file will be permanently removed. Continue?',
+          title: 'confirm.deleteTitle',
+          message: 'confirm.deleteMessage',
           danger: true,
         });
         if (!confirmed) return;
         await fetch('/api/files/' + id, { method: 'DELETE' });
-        this.showToast('File deleted');
+        this.showToast('toast.deleted');
         await this.loadFiles();
       },
 
@@ -70,7 +70,7 @@
         const data = await res.json();
         try {
           await navigator.clipboard.writeText(data.url);
-          this.showToast('Share link copied to clipboard');
+          this.showToast('toast.shareCopied');
         } catch (_) {
           window.prompt('Share URL:', data.url);
         }