style: add white glow outline to logo in dark mode

Use CSS drop-shadow filter on the logo img in dark mode to create a
subtle white outline that helps the transparent-background logo stand
out against the dark header and login page backgrounds.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/layout/AppLayout.tsx

@@ -106,7 +106,16 @@ export function AppLayout() {
         <Group h={60} px="md" justify="space-between">
           <Group>
             <Burger opened={opened} onClick={toggle} hiddenFrom="sm" size="sm" />
-            <img src={logoSrc} alt="HOA LedgerIQ" style={{ height: 40 }} />
+            <img
+              src={logoSrc}
+              alt="HOA LedgerIQ"
+              style={{
+                height: 40,
+                ...(colorScheme === 'dark' ? {
+                  filter: 'drop-shadow(0 0 1px rgba(255,255,255,0.8)) drop-shadow(0 0 2px rgba(255,255,255,0.4))',
+                } : {}),
+              }}
+            />
           </Group>
           <Group>
             {currentOrg && (