refactor molecule components to use .withProps() subcomponents and extract inline code

- Add UnstyledButton listItem theme variant to replace inline style for border-radius
- Extract subcomponent constants with .withProps() across all molecules
- Replace raw <span /> with Mantine <Flex /> in ResourcePreviewPanel
- Extract inline template literals and expressions to named functions

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

clients/web/src/theme/theme.ts

@@ -11,6 +11,7 @@ import {
   ThemeSelect,
   ThemeSwitch,
   ThemeTextInput,
+  ThemeUnstyledButton,
 } from "./index";
 
 export const theme = createTheme({
@@ -45,5 +46,6 @@ export const theme = createTheme({
     Select: ThemeSelect,
     Switch: ThemeSwitch,
     TextInput: ThemeTextInput,
+    UnstyledButton: ThemeUnstyledButton,
   },
 });