docs: mark Stage A UI/UX as implemented in Idea.md

[Neoplayer]

Motivation

• Clarify the current implementation status so the roadmap and todo items do not duplicate already shipped UI work.
• Provide concrete, code-linked evidence of which UI panels, context actions and visual behaviors are present in Stage A.

Description

• Added a new sub-point "UI/UX уже реализовано (уже в Stage A)" to 22.1) Текущее состояние listing implemented panels: Systems, NPC Corporations, Ship Card, Station Operations (Station Card), Markets, Finance, Policies, and Contracts.
• Documented implemented context actions Fly to station, Track ship, Open ship card / Open station card and the visual smoothing behavior via segment cache interpolation (segment_eta_remaining).
• Added code reference pointers to crates/gatebound_app/src/ui/hud/render.rs, crates/gatebound_app/src/runtime/sim.rs, and crates/gatebound_app/src/render/world.rs to serve as verification anchors.
• Clarified the 22.2) Ближайшие итерации (Stage A+) wording to mark station loop UX work as "polish only" for already implemented UI/UX.

Testing

• Verified the updated documentation snippet with sed -n '734,772p' docs/Idea.md to confirm the new subsection appears in place.
• Located corresponding UI code with rg -n across crates/gatebound_app/src/ui/hud/render.rs, crates/gatebound_app/src/runtime/sim.rs, and crates/gatebound_app/src/render/world.rs to validate referenced symbols and buttons were present.
• Confirmed the file update operation reported success via the patch application tool used to modify docs/Idea.md.
• Inspected the resulting file lines with nl -ba docs/Idea.md | sed -n '734,772p' to ensure formatting and placement are correct.

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Codex Task