feat: prepare project for cozy-ui migration

Build Configuration
- Add Stylus support via @rsbuild/plugin-stylus
- Add SVGR support via @rsbuild/plugin-svgr
- Update rsbuild.config.ts with custom configuration
- Add historyApiFallback for SPA routing

ESLint Configuration
- Create .eslintrc.json with react-app configuration
- Remove eslintConfig from package.json

CSS to Stylus Migration
Convert all 5 CSS files to Stylus (.styl) format:
  - src/App.css to src/App.styl
  - src/index.css to src/index.styl
  - src/components/Calendar/Calendar.css to src/components/Calendar/Calendar.styl
  - src/components/Calendar/CustomCalendar.css to src/components/Calendar/CustomCalendar.styl
  - src/components/Menubar/Menubar.css to src/components/Menubar/Menubar.styl
- Update all imports from .css to .styl
- Preserve all styling rules and values

MUI Styling Cleanup
- Replace all MUI sx props with standard style props across 15+ components
- Convert MUI spacing units (mr: 2 to marginRight: 16px)
- Convert MUI shorthand props (p: 2 to padding: 16px, mb: 1 to marginBottom: 8px)
- Remove MUI-specific syntax (&.Mui-checked selectors)
- Maintain exact visual appearance

FullCalendar Fixes
- Comment out problematic CSS rule in CustomCalendar.styl
- Fix display issues with timegrid slot labels

Files Modified
- Config: rsbuild.config.ts, .eslintrc.json, package.json
- Styling: 5 CSS to Stylus conversions, 15+ component sx to style updates
- Components: All components using MUI sx props updated

Development Workflow
- Continue using existing development commands
- Stylus files are now the source of truth for styling
- MUI components work with standard style props
- ESLint uses familiar react-app rules

Dependencies Added
- @rsbuild/plugin-stylus
- @rsbuild/plugin-svgr

src/components/Calendar/CalendarSelection.tsx

@@ -215,6 +215,7 @@ function CalendarSelector({
     handleClose();
   };
   return (
+<<<<<<< HEAD
     <>
       <div>
         <label>
@@ -252,5 +253,23 @@ function CalendarSelector({
         handleDeleteConfirm={handleDeleteConfirm}
       />
     </>
+=======
+    <div key={id}>
+      <label>
+        <Checkbox
+          style={{
+            color: calendars[id].color,
+          }}
+          size="small"
+          checked={selectedCalendars.includes(id)}
+          onChange={() => handleCalendarToggle(id)}
+        />
+        {calendars[id].name}
+      </label>
+      <IconButton onClick={() => setOpen()}>
+        <MoreVertIcon />
+      </IconButton>
+    </div>
+>>>>>>> a7e29ce (feat: prepare project for cozy-ui migration)
   );
 }