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/gitea_workspace.html

@@ -758,7 +758,7 @@ document.addEventListener('alpine:init', () => {
              x-show="gwCtx.visible"
              :style="'left:' + gwCtx.x + 'px; top:' + gwCtx.y + 'px'"
              @click.outside="gwCtx.visible = false"
-             style="display:none;position:fixed;background:var(--bg-secondary);border:1px solid var(--border-strong);border-radius:8px;box-shadow:0 8px 24px rgba(0,0,0,.4);z-index:1100;min-width:160px;padding:4px;">
+             style="display:none;position:fixed;background:var(--bg-modal);border:1px solid var(--border);border-radius:var(--radius-md);box-shadow:var(--shadow-popover);z-index:1100;min-width:160px;padding:4px;">
           <div class="ctx-item" @click="gwRename()">✏️ Rename</div>
           <div class="ctx-item ctx-danger" @click="gwDelete()">🗑 Delete</div>
         </div>