fix: apply shadcn violet OKLCH theme and fix SelectItem empty value crash

- Replace HSL-based indigo theme with official shadcn violet OKLCH theme
  in globals.css for proper contrast in both light and dark mode
- Update tailwind.config.ts to use var(--...) instead of hsl(var(--...))
  for OKLCH color space compatibility
- Fix Radix UI crash: replace SelectItem value="" with "__none__" sentinel
  in HostDialogs.tsx and L4HostDialogs.tsx (empty string value is invalid)
  Form action parsers already return null for non-numeric values

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

src/components/l4-proxy-hosts/L4HostDialogs.tsx

@@ -226,13 +226,13 @@ function L4HostForm({
         </Label>
         <Select
           name="proxy_protocol_version"
-          defaultValue={initialData?.proxy_protocol_version ?? ""}
+          defaultValue={initialData?.proxy_protocol_version ?? "__none__"}
         >
           <SelectTrigger id="proxy_protocol_version">
             <SelectValue placeholder="None" />
           </SelectTrigger>
           <SelectContent>
-            <SelectItem value="">None</SelectItem>
+            <SelectItem value="__none__">None</SelectItem>
             <SelectItem value="v1">v1</SelectItem>
             <SelectItem value="v2">v2</SelectItem>
           </SelectContent>