Fix mobile file picker (backdrop z-index bug)

Fixes the 'Choose char data file' button being greyed-out and unresponsive in mobile layout (and desktop browser resized to ≤700px).

Root cause: the desktop CSS rules for both drawer blades set z-index:200 after the mobile media query's z-index:1000 in the stylesheet. Equal-specificity rules resolve by source order, so the later desktop value always won — leaving the blades behind the backdrop (z-index:999) on mobile. The backdrop visually darkened the button and consumed all clicks (closing the blade instead of opening the file picker).

Fix: added !important to z-index:1000 in the mobile media query for both #file-select-blade and #blade.