Move org switcher to header, self-host Inter, add feature tags, normalize API route casing

- Replace sidebar org dropdown with a header link/menu (left of theme toggle) that
  includes a "New Organization" entry opening the create dialog
- Self-host Inter via @fontsource-variable/inter instead of relying on system fonts
- Wire up real feature<->tag assignment (was UI-only before): expose Tags on
  FeatureResponse, add assign/remove endpoints, make tag chips in the feature
  detail dialog toggle assignment, and show up to 5 tags (+ellipsis) in the
  features table next to the segment-override icon
- Normalize all API routes to singular/plural REST convention: singular resource
  name when addressing one item by id/key (e.g. /feature/{id}), plural for
  list/create endpoints (e.g. /features). Updated every controller, the admin
  frontend API clients, and affected tests to match

tests/api/MicCheck.Api.Tests.Unit/Features/FlagsApiIntegrationTests.cs

@@ -115,7 +115,7 @@ public class FlagsApiIntegrationTests
         var client = _factory.CreateClient();
         client.DefaultRequestHeaders.Add("X-Environment-Key", _envApiKey);
 
-        var response = await client.PostAsJsonAsync("/api/v1/identities/", new
+        var response = await client.PostAsJsonAsync("/api/v1/identity/", new
         {
             identifier = "user-123",
             traits = new[] { new { trait_key = "plan", trait_value = "premium" } }