Try to improve lighthouse score of servers demo

[sbernard31]

The lighthouse score of leshan-server-demo and leshan-bsserver-demo could be probably improved :

(full test can be done locally using chromium devTools or partial test online at : https://pagespeed.web.dev/ )

E.g. for leshan-server-demo (tested on sandbox : https://leshan.eclipseprojects.io)

[sbernard31]

Some hits about that : https://news.shardlabs.io/how-to-drastically-reduce-your-bundle-size-and-load-time-in-vue-js-54370d513fdf

[sbernard31]

few points performance point grabbed by #1242.

E.g. for leshan-server-demo (tested on sandbox : https://leshan.eclipseprojects.io/)

[sbernard31]

I retest https://leshan.eclipseprojects.io/ before/after https://github.com/eclipse-leshan/leshan/pull/1541.

Before Migration with vue-cli

For Mobile :

For Desktop :

After Migration with vite 5

For Mobile :

For Desktop :

Get some very little improve on performance.

[sbernard31]

Some score after #1549 :

For Mobile :

For Desktop :

(but score are not really constant and I see some significant changes from on test to another with same code targeted ... :expressionless: )

We can retest when we will move to vue3 + vuetify3