feat(ui): shared clickable-affordance accent for task-card rows

[Limdongcheng]

Summary

Task-card rows that expand or reveal content on click (processing step rows, CRAFT signal groups, region rows) rendered as plain text until hovered, so users never discovered they were clickable.

The Ribbon already has a clickable visual language — enabled button glyphs carry Visuals::hyperlink_color — so a new ui::affordance module exposes that colour (clickable_tint) plus a selectable_row helper that tints a row's leading glyph with it while the label keeps the theme text colour. The three click-to-enter rows now render through the helper; Statistics and Curve Fit cards already expose their actions through framed buttons and combo boxes, so they need no change. Scope is limited to the clickable-affordance semantic — no app-wide recolouring.

Validation

• cargo pr-check passes (fmt, source sizes, dependency policy, default-frontends build, clippy -D warnings, reference-backend test suite).
• A unit test pins clickable_tint to the Ribbon glyph colour in both themes so the visual language cannot silently fork.

UI display logic

• New or changed actions go through the command catalog (CommandId + describe) and are searchable in the command palette. (No actions added or changed — styling only.)
• Hiding happens only at Ribbon-group level (dataset kind or width budget); transient states disable with a disabled_reason that says how to unblock. (No show/hide logic touched.)
• No layout changes from background events; panels are never auto-closed. (No layout behavior touched.)
• New panels or empty regions show an empty state with a next step. (No new panels.)
• Docs updated in docs/ (English and zh-CN) for user-visible behavior. (Visual affordance only; no documented workflow changes.)