feat: admin can create users manually + disable registration (#85)

Backend:
- POST /api/admin/users — admin-only user creation endpoint
  - username & password required, email optional (auto-generated placeholder)
  - role, quota_bytes, active all configurable
  - creates personal folder automatically
- PUT /api/admin/users/{id}/password — admin password reset
- GET/PUT /api/admin/settings/registration — toggle public registration
  - Supports env var OXICLOUD_DISABLE_REGISTRATION override
  - Blocks POST /api/auth/register when disabled
- AdminCreateUserDto, AdminResetPasswordDto added to settings DTOs
- registration_enabled field added to DashboardStatsDto

Frontend (admin.html):
- 'Create User' button in Users tab with full modal form
  (username, password, email, role, quota)
- 'Reset Password' button per user in actions column
- 'Allow public self-registration' toggle in Dashboard > System
  with warning banner when disabled

Closes #85

src/application/dtos/settings_dto.rs

@@ -80,6 +80,27 @@ pub struct UpdateUserQuotaDto {
     pub quota_bytes: i64,
 }
 
+/// Request body for admin-created users
+#[derive(Debug, Serialize, Deserialize, Clone)]
+pub struct AdminCreateUserDto {
+    pub username: String,
+    pub password: String,
+    /// Optional — if omitted, a placeholder email is generated
+    pub email: Option<String>,
+    /// "admin" or "user"; defaults to "user"
+    pub role: Option<String>,
+    /// Storage quota in bytes; 0 = unlimited. If omitted, uses role default.
+    pub quota_bytes: Option<i64>,
+    /// Whether the account is active; defaults to true
+    pub active: Option<bool>,
+}
+
+/// Request body for admin password reset
+#[derive(Debug, Serialize, Deserialize)]
+pub struct AdminResetPasswordDto {
+    pub new_password: String,
+}
+
 /// Query parameters for listing users
 #[derive(Debug, Serialize, Deserialize)]
 pub struct ListUsersQueryDto {
@@ -105,4 +126,5 @@ pub struct DashboardStatsDto {
     pub storage_usage_percent: f64,
     pub users_over_80_percent: i64,
     pub users_over_quota: i64,
+    pub registration_enabled: bool,
 }