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

backend/internal/database/queries/queries.sql

@@ -1,6 +1,6 @@
 -- name: CreateUser :one
-INSERT INTO users (email, username, password_hash)
-VALUES ($1, $2, $3)
+INSERT INTO users (email, username, password_hash, theme)
+VALUES ($1, $2, $3, $4)
 RETURNING *;
 
 -- name: GetUserByEmail :one
@@ -13,7 +13,10 @@ SELECT * FROM users WHERE username = $1;
 SELECT * FROM users WHERE email = $1 OR username = $1;
 
 -- name: GetUser :one
-SELECT id, email, username, created_at, updated_at FROM users WHERE id = $1;
+SELECT id, email, username, theme, created_at, updated_at FROM users WHERE id = $1;
+
+-- name: ListUsers :many
+SELECT id, email, username, theme, created_at, updated_at FROM users ORDER BY created_at DESC;
 
 -- name: GetEbook :one
 SELECT * FROM ebooks WHERE id = $1;
@@ -54,4 +57,7 @@ DO UPDATE SET
 RETURNING *;
 
 -- name: DeleteReadingProgress :exec
-DELETE FROM reading_progress WHERE ebook_id = $1 AND user_id = $2;
+DELETE FROM reading_progress WHERE ebook_id = $1 AND user_id = $2;
+
+-- name: UpdateUserTheme :exec
+UPDATE users SET theme = $2, updated_at = NOW() WHERE id = $1;