docs: added sticky-header-technical-overview

[makhnatkin]

Summary by Sourcery

Documentation:

• Add a technical overview describing the architecture, behavior, contracts, constants, diagnostics, and constraints of the editor sticky-header mechanism.

[sourcery-ai]

Reviewer's Guide

Adds a new, detailed technical overview document describing the current sticky-header implementation in the editor, including its architecture, contracts, constants, and diagnostic scenarios.

File-Level Changes

Change	Details	Files
Document the current sticky-header implementation, its components, contracts, and diagnostics in a standalone technical spec.	Introduces a new markdown document capturing the end-to-end behavior and structure of the sticky-header mechanism in the editor. Describes the responsibilities and interactions of CSS sticky layout, React hooks (useSticky, useTargetZIndex), and getTopOffset for programmatic scrolling. Formalizes external props, CSS variables, DOM and CSS contracts, including invariants and behavioral rules around stickyToolbar, sticky-active, and popup z-index handling. Captures fixed constants (offsets, z-indexes, dimensions) and their cross-cutting relationships between CSS and JS implementations. Adds diagnostic guidance with control scenarios and common failure modes for sticky-header-related issues.	docs/sticky-header-technical-overview.md

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