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/querier.go

@@ -22,8 +22,10 @@ type Querier interface {
 	GetUserByEmailOrUsername(ctx context.Context, email string) (Users, error)
 	GetUserByUsername(ctx context.Context, username string) (Users, error)
 	ListEbooks(ctx context.Context, arg ListEbooksParams) ([]Ebooks, error)
+	ListUsers(ctx context.Context) ([]ListUsersRow, error)
 	UpdateEbook(ctx context.Context, arg UpdateEbookParams) (Ebooks, error)
 	UpdateReadingProgress(ctx context.Context, arg UpdateReadingProgressParams) (ReadingProgress, error)
+	UpdateUserTheme(ctx context.Context, arg UpdateUserThemeParams) error
 }
 
 var _ Querier = (*Queries)(nil)