Give App Shell and Document Layout regions the padding/background/border they never had

[KolesnikovKirill]

What and why

You pointed at a real screenshot: App Shell's sidebar and Document Layout's sidebar/aside had their content sitting flush against the edge, no visible panel around it at all.

Checked why rather than guessing at a fix — .cm-app-shell__sidebar, __aside, __content, __header, __subheader, and __footer, and the equivalent five regions in document-layout.css, had never had padding, and the sidebar/aside additionally never had a background or border. This predates #22 (just merged): that PR fixed positioning, responsiveness, and sticky behavior for both files but left this specific gap, because I verified those changes by checking for console warnings and correct attribute/behavior state, not by looking hard enough at what was actually on screen.

What changed

VueForge's fd793696 had exactly this chrome: .vf-sidebar-area / .vf-aside-area carried sidebarBackground/sidebarBorder and asidePaddingBlock/Inline, and .vf-content-area carried its own padding. Added the same shape here, using this package's own tokens rather than porting VueForge's token names:

• --cm-space-3 padding and --cm-color-background-surface on sidebar/aside/content
• border-inline-end on the sidebar and border-inline-start on the aside (--cm-color-border-subtle, matching every other border already in both files)
• --cm-space-2 --cm-space-3 padding on header/subheader/footer, matching the padding shorthand admin-layout.css and admin-shell.css already use for their own header/footer

Verified against the real thing

Screenshotted both layouts after rebuilding, at the viewport width from your screenshot, and confirmed sidebar/aside now render as distinct panels with visible backgrounds and borders, with their slot content no longer touching the edges.

Checks

npm run verify passes in full.