docs(ui): the Querya button and control standard

[ZhuchkaTriplesix]

Closes #1332 (part of epic #1331)

The standard, written before any component code so #1333-#1335 and the migration (#1337, #1338) implement and apply one set of rules. It lands in docs/ui-kit.md:

• One control scale: sm 28 / md 32 / lg 36, with icon size, label size, padding and gap per size; radius is one token; a hit area is never below sm.
• Variants: primary, secondary, ghost, destructive, as roles (no colour arguments).
• States: default, hover, pressed, keyboard-only focus ring, disabled, loading (width preserved).
• Layout rules: one size per row, split-button and menu-trigger composition, tooltip rules.
• Dialog footers: Cancel ghost first, exactly one primary / destructive last, Esc / Enter, canonical roles for Cancel / Close / Retry / Create.
• Mapping table from the shadcn and ad-hoc buttons to the new components, and the guard tests that land with chore(ui): remove the remaining shadcn buttons and add guard tests for the button standard #1339.

Docs only; no code changes. The Obsidian note named in the issue is outside the repo and is not touched here.

Decisions to confirm in review (they are numbers in the tables, cheap to change before #1333 turns them into tokens): sizes 28 / 32 / 36, icon sizes 15 / 16 / 18, radius 6, Retry as secondary.