shell: fix glitchy sidebar toggle animation (#132)

[ZoroLV]

Fixes #132

changes:

• refactored the sidebar to use fixed positioning with opacity based crossfades rather than constant unmount/remounting
• added the react 19's inert attribute to hide the inactive sidebar state from pointer events
• slightly increased main-content.tsx transition timing to duration-300 to better sync with the new sidebar animation

this eliminates the layout thrashing and text jitter from the container being resized

also by keeping both states in the DOM and animating opacity the browser doesn't need to trigger heavy reflows on every frame

before

before.mp4

after

after.mp4