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

[Limdongcheng]

Root cause

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.

Fix

The Ribbon already has a clickable visual language — enabled button glyphs carry Visuals::hyperlink_color. 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, so "this text is clickable" reads identically on every surface instead of each card inventing its own.

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 deliberately limited to the clickable-affordance semantic — no app-wide recolouring.

Tests

A unit test pins clickable_tint to the Ribbon glyph colour in both themes so the visual language cannot silently fork. cargo pr-check passes.

---

Part 2 of the stack, based on fix/task-card-geometry (#2).

[github-actions]

Thank you for your submission, we really appreciate it. Like many open-source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution. You can sign the CLA by just posting a Pull Request Comment same as the below format.

---

I have read the CLA Document and I hereby sign the CLA

---

_{You can retrigger this bot by commenting recheck in this Pull Request. Posted by the CLA Assistant Lite bot.}