Add admin/plans#index for side-by-side plan comparison

[Copilot]

Adds index view for comparing sponsorship plans in a table before publication.

Changes

• Controller: Added index action fetching plans ordered by rank; redirects after create/update now go to index instead of edit view
• Routes: Enabled GET /admin/conferences/:slug/plans
• View: Comparison table with rank, name, summary, price, capacity, guests, booth size, words limit, closes_at, auto_acceptance status (badged), and edit actions
• Integration: Added "View all plans" link from conference show page

The table layout:

%table.table.table-bordered.table-striped
  %thead
    %tr
      %th Rank | Name | Summary | Price | Capacity | ...
  %tbody
    - @plans.each do |plan|
      %tr
        %td= plan.rank
        %td= plan.name
        ...

Follows existing patterns from admin/announcements#index.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>create admin/plans#index to compare plans</issue_title>
<issue_description>create admin/plans#index action and view (along with existing new and create) with HTML table to compare created plans to allow humans to review before publication</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes create admin/plans#index to compare plans #103

---

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[github-actions]

Slowest examples

Top 10 slowest examples (6.24 seconds, 57.28% of total time)

Example	Description	Time in seconds
spec/jobs/create_broadcast_deliveries_job_spec.rb:42	CreateBroadcastDeliveriesJob filters CreateBroadcastDeliveriesJob::Filters::All status all is expected to eq [#<Sponsorship id: 1, accepted_at: "2025-11-21 08:45:33.911680000 +0000", booth_assigned: true, booth...98475000 +0000", url: "https://48.co.invalid", withdrawn_at: "2025-11-21 08:45:34.588543000 +0000">]	0.79775
spec/jobs/create_broadcast_deliveries_job_spec.rb:80	CreateBroadcastDeliveriesJob filters CreateBroadcastDeliveriesJob::Filters::All plan plan1 is expected to eq [#<Sponsorship id: 193, accepted_at: "2025-11-21 08:45:36.463767000 +0000", booth_assigned: true, boo...1517000 +0000", url: "https://232.co.invalid", withdrawn_at: "2025-11-21 08:45:36.946635000 +0000">]	0.61767
spec/jobs/create_broadcast_deliveries_job_spec.rb:111	CreateBroadcastDeliveriesJob filters CreateBroadcastDeliveriesJob::Filters::All exhibitor yes is expected to eq [#<Sponsorship id: 337, accepted_at: "2025-11-21 08:45:38.276675000 +0000", booth_assigned: true, boo...1407000 +0000", url: "https://382.co.invalid", withdrawn_at: "2025-11-21 08:45:38.826376000 +0000">]	0.61244
spec/jobs/create_broadcast_deliveries_job_spec.rb:122	CreateBroadcastDeliveriesJob filters CreateBroadcastDeliveriesJob::Filters::All composite ja pending plan is expected to eq [#<Sponsorship id: 405, accepted_at: nil, booth_assigned: true, booth_requested: false, conference_id...pdated_at: "2025-11-21 08:45:39.180171000 +0000", url: "https://408.co.invalid", withdrawn_at: nil>]	0.61046
spec/jobs/create_broadcast_deliveries_job_spec.rb:60	CreateBroadcastDeliveriesJob filters CreateBroadcastDeliveriesJob::Filters::All status pending is expected to eq [#<Sponsorship id: 113, accepted_at: nil, booth_assigned: true, booth_requested: false, conference_id...pdated_at: "2025-11-21 08:45:35.654234000 +0000", url: "https://128.co.invalid", withdrawn_at: nil>]	0.60428
spec/jobs/create_broadcast_deliveries_job_spec.rb:131	CreateBroadcastDeliveriesJob filters CreateBroadcastDeliveriesJob::Filters::All composite ja active plan is expected to eq [#<Sponsorship id: 445, accepted_at: "2025-11-21 08:45:39.650879000 +0000", booth_assigned: true, boo...pdated_at: "2025-11-21 08:45:39.693276000 +0000", url: "https://448.co.invalid", withdrawn_at: nil>]	0.60396
spec/jobs/create_broadcast_deliveries_job_spec.rb:140	CreateBroadcastDeliveriesJob filters CreateBroadcastDeliveriesJob::Filters::All composite ja accepted exhibitors is expected to eq [#<Sponsorship id: 485, accepted_at: "2025-11-21 08:45:40.151227000 +0000", booth_assigned: true, boo...pdated_at: "2025-11-21 08:45:40.267310000 +0000", url: "https://494.co.invalid", withdrawn_at: nil>]	0.60236
spec/jobs/create_broadcast_deliveries_job_spec.rb:51	CreateBroadcastDeliveriesJob filters CreateBroadcastDeliveriesJob::Filters::All status active is expected to eq [#<Sponsorship id: 49, accepted_at: "2025-11-21 08:45:34.671339000 +0000", booth_assigned: true, boot...updated_at: "2025-11-21 08:45:34.857812000 +0000", url: "https://64.co.invalid", withdrawn_at: nil>]	0.60087
spec/jobs/create_broadcast_deliveries_job_spec.rb:91	CreateBroadcastDeliveriesJob filters CreateBroadcastDeliveriesJob::Filters::All locale ja is expected to eq [#<Sponsorship id: 245, accepted_at: "2025-11-21 08:45:37.130920000 +0000", booth_assigned: true, boo...5586000 +0000", url: "https://288.co.invalid", withdrawn_at: "2025-11-21 08:45:37.640762000 +0000">]	0.59706
spec/jobs/create_broadcast_deliveries_job_spec.rb:100	CreateBroadcastDeliveriesJob filters CreateBroadcastDeliveriesJob::Filters::All locale en is expected to eq [#<Sponsorship id: 289, accepted_at: "2025-11-21 08:45:37.680010000 +0000", booth_assigned: true, boo...8464000 +0000", url: "https://332.co.invalid", withdrawn_at: "2025-11-21 08:45:38.193848000 +0000">]	0.59684