Please ! Be aware !

NEVER RUN ONLINE SERVER (available from internet) IN DEV MODE --dev ! (not secure since console available in traceback)

2 distinct thing:

dev mode (--dev cli) server which réad xml, show console in traceback, ...

debug mode  (?debug=1 in url) which display all technicals features, but don't load asset but each file one by one !

take a look at my first comment, I continue to think that it the simple way and free to do it !

So create a view with inherit view of 'login' and mode 'extented view' with xpath to update redirect input

<xpath expr="//input[@name='redirect']" position='attributes'>
    <attribute name="t-att-value">'%s%sdebug=1' % (redirect or '/web', redirect and '?' in redirect and '&amp;' or '?')</attribute>
</xpath>

Hi,

You can use the "Automatic Developer Mode" module available in the Odoo app store to activate the developer mode automatically after the login action in Odoo 9.

Here is the link to the app 

https://apps.odoo.com/apps/modules/9.0/developer_mode/

Regards

Its not automatically but here chrome extension so in just one click you can activate debug mode. http://pinakinnayi.blogspot.in/2016/02/activate-debug-mode-in-one-click.html

Hello

please set option 

In windows 

openerp.config file

debug= true