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/Admin/AdminApiIntegrationTests.cs

@@ -97,7 +97,7 @@ public class AdminApiIntegrationTests
         db.Projects.Add(project);
         db.SaveChanges();
 
-        var response = await client.PostAsJsonAsync($"/api/v1/projects/{project.Id}/features", new
+        var response = await client.PostAsJsonAsync($"/api/v1/project/{project.Id}/features", new
         {
             name = "dark_mode",
             type = "Standard",
@@ -135,7 +135,7 @@ public class AdminApiIntegrationTests
         });
         db.SaveChanges();
 
-        await client.PostAsJsonAsync($"/api/v1/projects/{project.Id}/features", new
+        await client.PostAsJsonAsync($"/api/v1/project/{project.Id}/features", new
         {
             name = "flag_x",
             type = "Standard",
@@ -220,7 +220,7 @@ public class AdminApiIntegrationTests
         db.Projects.Add(project);
         db.SaveChanges();
 
-        var response = await client.PostAsJsonAsync($"/api/v1/projects/{project.Id}/features", new
+        var response = await client.PostAsJsonAsync($"/api/v1/project/{project.Id}/features", new
         {
             name = "invalid name with spaces!",
             type = "Standard"
@@ -247,7 +247,7 @@ public class AdminApiIntegrationTests
         db.Projects.Add(project);
         db.SaveChanges();
 
-        var response = await client.PostAsJsonAsync($"/api/v1/projects/{project.Id}/segments", new
+        var response = await client.PostAsJsonAsync($"/api/v1/project/{project.Id}/segments", new
         {
             name = "Premium Users",
             rules = new[]