fix(studio): give flat-inspector value fields a resting-state input affordance

Every editable value in the flat inspector (FlatRow's CommitField, the
Motion Timing row's Start/End/Duration cells, and every raw <select> —
Style/Text dropdowns, Grade's Custom LUT and Copy-grade-to scope) rendered
its underline/border only on hover (`border-transparent
group-hover:border-...` or no border at all). At rest a value looked like
plain static text, with nothing distinguishing it from a label — testers
reported not being able to tell which fields were editable.

Give each a dim-but-visible resting border (`border-panel-border-input/50`,
or `border-panel-accent/30` for the explicitCustom tier) that brightens on
hover/focus, instead of a fully transparent one. Purely visual — no
behavior change.

Full studio suite (2641 tests) green; typecheck/oxlint/oxfmt clean.

packages/studio/src/components/editor/propertyPanelFlatTextSection.tsx

@@ -111,7 +111,7 @@ function FlatTextFieldEditor({
         >
           Weight
         </span>
-        <label className="flex items-center gap-1.5">
+        <label className="flex items-center gap-1.5 border-b border-panel-border-input/50 pb-px hover:border-panel-border-input">
           <select
             value={weight}
             onChange={(e) => {