docs: document subtle variant for both Button and ActionIcon

Update code_style.md to reflect that variant="subtle" is accepted for
both Button and ActionIcon, not just ActionIcon.

Made-with: Cursor

Author: elizabetdev

agent_docs/code_style.md

@@ -37,14 +37,15 @@ The project uses Mantine UI with **custom variants** defined in `packages/app/sr
 | `variant="secondary"` | Secondary actions (Cancel, Clear, auxiliary actions) | `<Button variant="secondary">Cancel</Button>` |
 | `variant="danger"` | Destructive actions (Delete, Remove, Rotate API Key) | `<Button variant="danger">Delete</Button>` |
 | `variant="link"` | Link-style actions with no background or border (View Details, navigation-style CTAs) | `<Button variant="link">View Details</Button>` |
-| `variant="subtle"` | **ActionIcon only.** Transparent background with hover highlight; for toolbar/utility icons that shouldn't draw attention until hovered (collapse toggles, close buttons, auxiliary controls) | `<ActionIcon variant="subtle">...</ActionIcon>` |
+| `variant="subtle"` | Transparent background with hover highlight; for toolbar/utility controls that shouldn't draw attention until hovered (collapse toggles, close buttons, auxiliary actions) | `<Button variant="subtle">Filter</Button>` |
 
 ### Correct Usage
 
 ```tsx
 <Button variant="primary">Save</Button>
 <Button variant="secondary">Cancel</Button>
 <Button variant="danger">Delete</Button>
+<Button variant="subtle">Filter</Button>
 <Button variant="link">View Details</Button>
 <ActionIcon variant="primary">...</ActionIcon>
 <ActionIcon variant="secondary">...</ActionIcon>
@@ -70,7 +71,7 @@ The project uses Mantine UI with **custom variants** defined in `packages/app/sr
 
 **Link variant details**: Renders with no background, no border, and muted text color. On hover, text brightens to full contrast. Use for link-style CTAs that should blend into surrounding content (e.g., "View Details", "View Full Trace").
 
-**Subtle variant details (ActionIcon only)**: Transparent background with standard text color. On hover, a subtle background highlight appears (`--color-bg-hover`). This is the **default** ActionIcon variant. Use for toolbar icons, collapse toggles, close buttons, and utility controls that should stay unobtrusive but reveal interactivity on hover. Unlike `link`, `subtle` shows a hover background rather than changing text color.
+**Subtle variant details**: Transparent background with standard text color. On hover, a subtle background highlight appears (`--color-bg-hover`). This is the **default** ActionIcon variant. Use for toolbar icons, collapse toggles, close buttons, and utility controls that should stay unobtrusive but reveal interactivity on hover. Unlike `link`, `subtle` shows a hover background rather than changing text color.
 
 **Note**: `variant="filled"` is still valid for **form inputs** (Select, TextInput, etc.), just not for Button/ActionIcon.