Extract Engine::picker_scroll(delta) from backend scroll handlers

[JDonaghy]

Problem

Both backends directly manipulate engine.picker_selected and engine.picker_scroll_top with manual clamping, then call engine.picker_load_preview():

• GTK: mod.rs:4471-4489, ~19 lines
• TUI: mouse.rs:617-868, ~30 lines of scroll logic (within a larger picker block)

Proposed fix

Add Engine::picker_scroll(delta: isize) that handles selection movement, scroll offset clamping, and preview loading in one call.

Context

Identified in Session 370 backend dedup audit.