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/library.html

@@ -110,7 +110,7 @@
 .source-filter-btn.active {
   background: var(--bg-primary);
   color: var(--text-primary);
-  box-shadow: 0 1px 2px rgba(0,0,0,.2);
+  box-shadow: var(--shadow-popover);
 }
 
 /* ── Card grid ── */
@@ -171,15 +171,15 @@
 }
 .source-local {
   background: rgba(100, 180, 100, 0.15);
-  color: #64b464;
+  color: var(--green);
 }
 .source-gitea {
   background: rgba(100, 160, 220, 0.15);
-  color: #64a0dc;
+  color: var(--blue);
 }
 .source-github {
   background: rgba(180, 140, 220, 0.15);
-  color: #b48cdc;
+  color: var(--purple);
 }
 .library-card-workspace {
   font-size: 12px;