feat(ui): z-index mini framework via Tailwind custom tiers

[enkr1]

Addresses #5397

Problem

20+ hardcoded z-index values scattered across the codebase with no convention — values range from 1 to 9999 with no semantic meaning or hierarchy.

Changes

Tailwind z-index tiers (tailwind.config.ts)

Tier	Class	Value	Usage
base	z-base	1	Base layer elements
badge	z-badge	3	Badges, icons overlaying content
sticky	z-sticky	100	Sticky headers, controls
overlay	z-overlay	110	Loading overlays
dropdown	z-dropdown	1000	Menus, popovers, floating controls
modal	z-modal	9999	Dialogs, notifications — always on top

Migrated components

• NotificationBar — 9999 → z-modal
• ConfirmationDialog — 9999 → z-modal
• LoadingOverlay — 110 → z-overlay
• ZoomActionElements — 999 → z-dropdown
• Dashboard (learning map) — 999 → z-dropdown
• UnpublishedIcon — 3 → z-badge

Not migrated (intentionally)

• CKEditor CSS (z-index: 1500) — third-party DOM, can't use Tailwind classes
• Learning map Node/Gate — dynamic z-index computed from node.depth, must stay inline
• ScribingView fields — string '1' values in inline styles, minor cleanup for a follow-up
• WideComments — dynamic z-index based on line numbers

Reference

Smashing Magazine: Z-Index for Large Projects