Favicon View Display with Fallback Placeholder

What Was Built

• Added source_favicon_tag helper rendering favicon image (when attached) or colored-circle initials placeholder
• Placeholder uses first letter of source name with HSL color derived from name bytes for consistent coloring
• ActiveStorage guard pattern: favicon_attached? checks defined?(ActiveStorage), respond_to?(:favicon), and attached?
• Sources index row now shows 24px favicon next to source name in flex layout
• Source show page shows 40px favicon next to name heading
• Task 4 (import sessions) confirmed as no-op: OPML preview sources don't have favicons

Files Modified

• app/helpers/source_monitor/application_helper.rb -- added source_favicon_tag, favicon_attached?, favicon_image_tag, favicon_placeholder_tag
• app/views/source_monitor/sources/_row.html.erb -- wrapped name cell in flex container with favicon tag
• app/views/source_monitor/sources/_details.html.erb -- added favicon next to h1 heading
• test/helpers/source_monitor/favicon_helper_test.rb -- 13 tests covering placeholders, sizing, colors, edge cases
• test/system/sources_test.rb -- updated assert_source_order to match link text (accommodates favicon initial)