web: Refactor frontend utilities and eliminate duplicate code

Extract shared test utilities (getStatusColor, getStatusKey, formatDuration)
into web/src/lib/utils/test.ts and extend datetime.ts with formatTestDate and
formatRelative (with full i18n support). Remove 4 sets of duplicate functions
across the tests/* pages, fix $app/stores → $app/state in plugins/[pid], and
fix inline type import in results/[rid].

Create PluginOptionsGroups.svelte shared component for read-only option group
rendering, used by both web and web-admin plugin detail pages. Update
web-admin/src/routes/plugins/[pname]/+page.svelte to use i18n translations
throughout, replacing all hardcoded English strings.

Add i18n keys: tests.relative.{in,ago,just-now,in-less-than-a-minute} and
plugins.tests.back-button to en.json and fr.json.

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

internal/api-admin/controller/plugin_test_controller.go

@@ -74,6 +74,37 @@ func (uc *TestPluginController) TestPluginOptionHandler(c *gin.Context) {
 	c.Next()
 }
 
+// ListTestPlugins retrieves all available test plugins.
+//
+//	@Summary		List test plugins (admin)
+//	@Schemes
+//	@Description	Retrieves a list of all available test plugins with their version information.
+//	@Tags			plugins
+//	@Accept			json
+//	@Produce		json
+//	@Success		200	{object}	map[string]happydns.PluginVersionInfo	"Map of plugin name to version info"
+//	@Failure		500	{object}	happydns.ErrorResponse					"Internal server error"
+//	@Router			/plugins/tests [get]
+func (uc *TestPluginController) ListTestPlugins(c *gin.Context) {
+	uc.BaseTestPluginController.ListTestPlugins(c)
+}
+
+// GetTestPluginStatus retrieves the status and available options for a test plugin.
+//
+//	@Summary		Get test plugin status (admin)
+//	@Schemes
+//	@Description	Retrieves the status and available configuration options for a specific test plugin.
+//	@Tags			plugins
+//	@Accept			json
+//	@Produce		json
+//	@Param			pname	path		string					true	"Plugin name"
+//	@Success		200		{object}	happydns.PluginStatus	"Plugin status with available options"
+//	@Failure		404		{object}	happydns.ErrorResponse	"Plugin not found"
+//	@Router			/plugins/tests/{pname} [get]
+func (uc *TestPluginController) GetTestPluginStatus(c *gin.Context) {
+	uc.BaseTestPluginController.GetTestPluginStatus(c)
+}
+
 // GetTestPluginOptions retrieves all admin-level options for a test plugin.
 //
 //	@Summary		Get test plugin options (admin)