feat: add platform configuration tables and admin settings

Add four DB-backed configuration tables for site-wide settings:
site_settings (logo, top-left title, header/footer text with zh/en
variants), upload_configs (master switch, default size, storage dir),
upload_file_types (per-extension whitelist with per-type size limits),
and download_baseurls (multiple download sources with priority/default).

- Models, AutoMigrate, and first-run seed (18 common file types)
- Process-level config cache warmed at startup, refreshed on admin save
- SetUserContext injects cached site settings into templates
- base.html renders logo (local upload or external URL), title, header
  banner, and footer with i18n fallback
- Upload validator drives avatar uploads (form + AJAX) through the
  configured switch/type/size policy
- Admin pages for site/upload/download settings with i18n keys

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

handlers/helpers.go

@@ -16,16 +16,28 @@ func DefaultData(c *gin.Context) gin.H {
 	avatar, _ := c.Get("avatar")
 	displayName, _ := c.Get("display_name")
 	role, _ := c.Get("role")
+	siteSetting, _ := c.Get("site_setting")
+	siteLogo, _ := c.Get("site_logo")
+	siteLogoIsURL, _ := c.Get("site_logo_is_url")
+	siteLogoText, _ := c.Get("site_logo_text")
+	siteHeaderText, _ := c.Get("site_header_text")
+	siteFooterText, _ := c.Get("site_footer_text")
 
 	return gin.H{
-		"Tr":          tr,
-		"IsLoggedIn":  isLoggedIn,
-		"Username":    username,
-		"Lang":        lang,
-		"SwitchLang":  switchLang,
-		"Avatar":      avatar,
-		"DisplayName": displayName,
-		"Role":        role,
+		"Tr":               tr,
+		"IsLoggedIn":       isLoggedIn,
+		"Username":         username,
+		"Lang":             lang,
+		"SwitchLang":       switchLang,
+		"Avatar":           avatar,
+		"DisplayName":      displayName,
+		"Role":             role,
+		"SiteSetting":      siteSetting,
+		"SiteLogo":         siteLogo,
+		"SiteLogoIsURL":    siteLogoIsURL,
+		"SiteLogoText":     siteLogoText,
+		"SiteHeaderText":   siteHeaderText,
+		"SiteFooterText":   siteFooterText,
 	}
 }