Extract render::compute_editor_layout() to eliminate repeated layout arithmetic

[JDonaghy]

Problem

The same editor layout arithmetic (subtract tab bar, quickfix, terminal, debug toolbar, status bar from available space) is computed in multiple places:

• GTK draw.rs:122-202 (~80 lines) — draw_editor
• GTK mod.rs sync_scrollbar + sync_scrollbar_positions (~300 lines combined)
• GTK scroll handler, overlay drag handlers
• TUI build_screen_for_tui (~84 lines)
• TUI viewport sync in mod.rs (~50 lines)
• TUI mouse bottom-chrome calculation

These must all agree, and when a new chrome element is added (e.g. breadcrumb bar), every copy must be updated.

Proposed fix

Add render::compute_editor_layout(engine, width, height, line_height) -> EditorLayout that returns a struct with all computed regions (tab_bar_rect, editor_rect, quickfix_rect, terminal_rect, status_rect, etc.). All consumers use this one computation.

Context

Identified in Session 370 backend dedup audit (Tier 2).