improve: 対象期間セレクトボックスの変更で自動遷移（表示ボタン削除）

- onchangeイベントで選択と同時にページ遷移
- 「表示」ボタンを削除してUIをシンプルに
- form_withタグも不要になったため削除
- より直感的で素早い操作が可能に

Author: yasulab

app/views/dojos/index.html.erb

@@ -37,28 +37,23 @@
       </div>
     <% end %>
 
-    <%= form_with(url: dojos_path, method: :get, local: true, style: 'margin-top: 15px;') do |f| %>
-      <div style="display: flex; align-items: center; justify-content: center; gap: 10px;">
-        <%= label_tag :year, '対象期間:', style: 'font-weight: bold;' %>
-        <%= select_tag :year, 
-            options_for_select(
-              (2012..Date.current.year).to_a.reverse.map { |y| [y.to_s + '年', y] },
-              params[:year]
-            ),
-            include_blank: '全期間',
-            style: 'padding: 5px; border: 1px solid #ced4da; border-radius: 4px;' %>
-        
-        <%= button_tag type: 'submit', style: 'padding: 5px 15px; background: #17a2b8; color: white; border: none; border-radius: 4px; cursor: pointer;' do %>
-          表示
-        <% end %>
-        
-        <%= link_to 'CSV', dojos_path(format: :csv, year: params[:year]), 
-            style: 'padding: 5px 15px; background: #28a745; color: white; text-decoration: none; border-radius: 4px;' %>
-        
-        <%= link_to 'JSON', dojos_path(format: :json, year: params[:year]), 
-            style: 'padding: 5px 15px; background: #6c757d; color: white; text-decoration: none; border-radius: 4px;' %>
-      </div>
-    <% end %>
+    <div style="display: flex; align-items: center; justify-content: center; gap: 10px; margin-top: 15px;">
+      <%= label_tag :year, '対象期間:', style: 'font-weight: bold;' %>
+      <%= select_tag :year, 
+          options_for_select(
+            (2012..Date.current.year).to_a.reverse.map { |y| [y.to_s + '年', y] },
+            params[:year]
+          ),
+          include_blank: '全期間',
+          onchange: "window.location.href = '#{dojos_path}?year=' + this.value",
+          style: 'padding: 5px; border: 1px solid #ced4da; border-radius: 4px; cursor: pointer;' %>
+      
+      <%= link_to 'CSV', dojos_path(format: :csv, year: params[:year]), 
+          style: 'padding: 5px 15px; background: #28a745; color: white; text-decoration: none; border-radius: 4px;' %>
+      
+      <%= link_to 'JSON', dojos_path(format: :json, year: params[:year]), 
+          style: 'padding: 5px 15px; background: #6c757d; color: white; text-decoration: none; border-radius: 4px;' %>
+    </div>
 
     <p style="font-size: smaller; color: #6c757d; margin-top: 15px;">
       対象期間を選択すると、その時点のアクティブな道場の一覧を表示・ダウンロードできます。

spec/requests/dojos_spec.rb

@@ -240,10 +240,11 @@
     end
 
     describe "HTML format year selection UI" do
-      it "shows year selection form" do
+      it "shows year selection form with auto-submit" do
         get dojos_path
         expect(response.body).to include('対象期間')
         expect(response.body).to include('<select')
+        expect(response.body).to include('onchange')
         expect(response.body).to include('2012')
         expect(response.body).to include(Date.current.year.to_s)
       end