Add user theme support and migrate frontend to HTMX templates

- Add theme column to users table with default 'tokyo-night'
- Update all user queries to include theme field
- Add ListUsers and UpdateUserTheme database queries
- Update auth handlers to support HTMX form submissions and JSON API
- Add ListUsers API endpoint
- Replace embedded static files with Go templates
- Update Dockerfile to copy templates directory
- Redesign index.html with inline styles and HTMX forms
- Update Bruno API testing requests for auth endpoints

bruno/auth/List Users.bru

@@ -0,0 +1,40 @@
+meta {
+  name: List Users
+  type: http
+  seq: 5
+}
+
+get {
+  url: {{base_url}}/api/users
+  body: none
+  auth: inherit
+}
+
+settings {
+  encodeUrl: true
+  timeout: 0
+}
+
+docs {
+  ## List Users
+  
+  Retrieves a list of all users.
+  
+  **Method:** GET
+  
+  **Endpoint:** /api/users
+  
+  **Authentication:** Required
+  
+  **Response:** Array of user objects
+  - `id` (string): User ID
+  - `email` (string): Email
+  - `username` (string): Username
+  - `theme` (string): User theme preference
+  - `created_at` (string): Creation timestamp
+  - `updated_at` (string): Last update timestamp
+  
+  **Status Codes:**
+  - 200: Success
+  - 401: Unauthorized
+}