fix(richtext-lexical): prevent upload media overflow on narrow viewports

[nehaaprasad]

What?

• Upload block media no longer overflow on small screens or in narrow columns (e.g. 50% width).

Why?

• A fixed min-width/min-height of 450px stopped media from shrinking and caused overflow.

How?

• Replaced those with max-width: min(450px, 100%) (and the same idea for portrait), and set max-width: 100% on the upload block and its contents so media scale with the container.

Fixes : #15559