Add self-service event creation from nodes#show

nodes#show's events table now renders unconditionally (previously
hidden entirely for a zero-event node) and gains an "add event" link.
The suggested tag_list is derived from the page's own category tags
via NodesHelper::DEFAULT_EVENT_TAG_BY_PAGE_TAG (erfa-detail/
chaostreff-detail -> open-day) rather than hardcoded, and degrades to
a blank field for any node whose tags don't match - deliberately
universal, not chapter-specific, since Updates have historically
carried event dates the same way.

events#new surfaces *why* the tag was pre-filled via flash.now (not
flash - this is a same-request render, not a redirect), using an
explicit auto_tag_source param passed alongside tag_list rather than
having the controller re-derive the reason from node_id.

No destroy link added to nodes#show's events list - deliberate, per
existing subnav-semantics convention (destructive actions live on the
resource's own views). Covered directly by test.

Adds real coverage for EventsController, previously 100% commented-out
scaffold, plus unit tests for the new tag-mapping helper and the
weekday-abbreviation helpers from the prior commit.

1 files changed, 4 insertions, 0 deletions

diff --git a/app/controllers/events_controller.rb b/app/controllers/events_controller.rb
index 9bed5dd..b98a38e 100644
--- a/app/controllers/events_controller.rb
+++ b/app/controllers/events_controller.rb
@@ -37,6 +37,10 @@ class EventsController < ApplicationController
       tag_list: params[:tag_list]
     )
 
+    if params[:tag_list].present? && params[:auto_tag_source].present?
+      flash.now[:notice] = "Tag '#{params[:tag_list]}' was pre-filled because this page is tagged '#{params[:auto_tag_source]}'. You can remove it below."
+    end
+
     respond_to do |format|
       format.html # new.html.erb
       format.xml  { render :xml => @event }