fix: Windows path handling, light theme default, file viewer in editor, first-user admin logic

Config:
- Fixed SQLite path parsing on Windows (handle drive letters without prepending /)
- Changed default theme from dark to light
- Updated OAuth test credentials (gitea_oauth_client_id/secret)

Auth:
- First real user (excluding default admin with no password) becomes admin
- Changed user count query to exclude users without password_hash

File viewer:
- Removed separate _render_file_viewer() —

app/templates/trash.html

@@ -22,7 +22,7 @@
   <!-- Filters -->
   <div style="display:flex; gap:8px; margin-bottom:16px;">
     <button class="trash-filter active">
-      <span style="color:#5b9bd5;">👤</span> Last edited by ▾
+      <span style="color:var(--accent);">👤</span> Last edited by ▾
     </button>
     <button class="trash-filter">
       <span>📁</span> In ▾