feat: complete Syndarix rebranding from PragmaStack

- Update PROJECT_NAME to Syndarix in backend config
- Update all frontend components with Syndarix branding
- Replace all GitHub URLs with Gitea Syndarix repo URLs
- Update metadata, headers, footers with new branding
- Update tests to match new URLs
- Update E2E tests for new repo references
- Preserve "Built on PragmaStack" attribution in docs

Closes #13

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/docs/branding/02-visual-identity.md

@@ -4,10 +4,10 @@
 
 ## Logo
 
-The **PragmaStack** logo represents the core values of the project: structure, speed, and clarity.
+The **Syndarix** logo represents the core values of the project: structure, speed, and clarity.
 
 <div align="center">
-  <img src="../../public/logo.svg" alt="PragmaStack Logo" width="300" />
+  <img src="../../public/logo.svg" alt="Syndarix Logo" width="300" />
   <p><em>The Stack: Geometric layers representing the full-stack architecture.</em></p>
 </div>
 
@@ -16,7 +16,7 @@ The **PragmaStack** logo represents the core values of the project: structure, s
 For smaller contexts (favicons, headers), we use the simplified icon:
 
 <div align="center">
-  <img src="../../public/logo-icon.svg" alt="PragmaStack Icon" width="64" />
+  <img src="../../public/logo-icon.svg" alt="Syndarix Icon" width="64" />
 </div>
 
 For now, we use the **Lucide React** icon set for all iconography. Icons should be used sparingly and meaningfully to enhance understanding, not just for decoration.