Introduce central z-index register with semantic variables in core

[dopenguin]

Description

A central register of semantic z-index css custom properties named --polar-z-index-* on the :host of PolarContainer.ce.vue should be introduced to make layering of UI elements easier and less confusing.

Suggested roles are the following while values open during implementation:

• --polar-z-index-ui-background
• --polar-z-index-ui-foreground
• --polar-z-index-context-menu
• --polar-z-index-toasts
• --polar-z-index-overlay
• --polar-z-index-tooltip

Current behavior

~18–20 z-index values are scattered across core and plugin components, making layering hard to verify and conflict-prone. Values are hardcoded per component with no shared source.

Expected behavior

A single documented set of semantic --polar-z-index-* variables on the core :host. All components consume them via var(--polar-z-index-*). Layer changes happen in one place, eliminating magic numbers. Concrete numeric values are proposals and may be finalized during implementation.

Affected areas

No response

Out of scope

• OpenLayers feature zIndex (e.g. in src/plugins/geoLocation/store.ts, src/plugins/pins/store.ts). This controls map-layer stacking, not UI layering, and is explicitly excluded.
• No visual redesign. Equivalent layering via variables only. Final numeric values may be adjusted during implementation.