fix(ui): add min-height and padding to textarea field (#16442)

Fixes textarea field shrinking below a minimum height when content is
typed and adds proper padding.

### Changes
- Adds `padding: var(--spacer-2)` to textarea for consistent 8px padding
(matching Figma design)
- Removes the `:not(:empty)` selector that was overriding min-height
with a rows-based calculation that could be smaller than the base
min-height or invalid when `--rows` wasn't set

### Before
Textarea would shrink to content size as you typed, potentially becoming
very small.

### After
Textarea maintains a minimum height of `3.75rem` and grows with content
via `field-sizing: content`.

Author: JarrodMFlesch

packages/ui/src/fields/Textarea/index.css

@@ -18,6 +18,7 @@
       height: auto;
       display: flex;
       field-sizing: content;
+      padding: var(--spacer-2);
 
       &:hover:not(:focus):not(:disabled) {
         border-color: var(--border-default-default);
@@ -28,10 +29,6 @@
       }
     }
 
-    textarea:not(:empty) {
-      min-height: calc(var(--rows) * var(--base) + 1rem + 0.125rem);
-    }
-
     &.read-only {
       textarea {
         background: var(--bg-default-default);