feat(editor): smart unit suffix — dim on focus, yield to explicit units

While editing a numeric field, the unit suffix now visually responds to
the draft content:
- Plain number / empty: suffix dims to 40% opacity (signals "default unit
  if you commit a bare number")
- Explicit unit typed (hint showing): suffix fades to opacity-0 over
  150ms (the conversion hint provides unit feedback instead)
- Unparseable text: suffix stays dimmed (field will revert on commit)
- At rest: unchanged full-opacity suffix

Uses opacity-0 rather than unmounting so the flex layout doesn't jump.
The predicate is simply `hint !== null` — it aligns exactly with
measurementHint's internal PLAIN_NUMBER guard.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: Aymericr

packages/editor/src/components/ui/controls/metric-control.tsx

@@ -330,7 +330,16 @@ export function MetricControl({
               type="text"
               value={inputValue}
             />
-            {displayUnit && <span className="ml-[1px] text-muted-foreground">{displayUnit}</span>}
+            {displayUnit && (
+              <span
+                className={cn(
+                  'ml-[1px] transition-opacity duration-150',
+                  hint ? 'opacity-0' : 'text-muted-foreground/40',
+                )}
+              >
+                {displayUnit}
+              </span>
+            )}
           </div>
         ) : (
           <div

packages/editor/src/components/ui/controls/slider-control.tsx

@@ -329,7 +329,16 @@ export function SliderControl({
               type="text"
               value={inputValue}
             />
-            {displayUnit && <span className="ml-[1px] text-muted-foreground">{displayUnit}</span>}
+            {displayUnit && (
+              <span
+                className={cn(
+                  'ml-[1px] transition-opacity duration-150',
+                  hint ? 'opacity-0' : 'text-muted-foreground/40',
+                )}
+              >
+                {displayUnit}
+              </span>
+            )}
           </>
         ) : (
           <div