web-app-taiga: refactor OIDC gating + defaults

- Introduced dedicated variables in vars/main.yml:
  * TAIGA_FLAVOR_TAIGAIO
  * TAIGA_TAIGAIO_ENABLED
- Replaced inline Jinja2 get_app_conf checks with TAIGA_TAIGAIO_ENABLED for
  consistency in tasks, docker-compose template and env file.
- Adjusted env.j2 to use TAIGA_TAIGAIO_ENABLED instead of direct flavor checks.
- Enabled css by default (true instead of false).
- Cleaned up spacing/indentation in config and env.

This improves readability, reduces duplicated logic, and makes it easier to
maintain both OIDC flavors (robrotheram, taigaio).

Conversation: https://chatgpt.com/share/68af65b3-27c0-800f-964f-ff4f2d96ff5d

roles/web-app-taiga/config/main.yml

@@ -7,17 +7,17 @@ oidc: {}
 flavor:               'taigaio' # Potential flavors: robrotheram, taigaio
 features:
   matomo:             true
-  css:                false
-  desktop:    true
+  css:                true
+  desktop:            true
   oidc:               false
   central_database:   true
   logout:             true
 docker:
   services:
     database:
-      enabled: true
+      enabled:        true
     taiga: 
-      version: "latest"
+      version:        "latest"
 server:
   csp:
     flags: