Fix #76: Remove invisible involvement state to prevent rooms from #76

[adtruiz]

Problem

When clicking the ⭐ icon to unstar a room from the room view, it would incorrectly remove the room from both "My Rooms" AND
"All Rooms". This was caused by a legacy "hide rooms" feature from Campfire that used an 'invisible' involvement state.

Solution

Removed the 'invisible' state from the involvement cycling for shared rooms, making both sidebar and room view use the same
2-state cycle: mentions ↔ everything

Changes Made

Code Changes

• app/helpers/rooms/involvements_helper.rb - Removed invisible from involvement cycling
• app/controllers/rooms/involvements_controller.rb - Fixed controller logic that was broadcasting room removal from
  sidebar
• app/models/rooms/thread.rb - Updated default involvement from invisible to mentions
• app/assets/images/notification-bell-invisible.svg - Deleted unused icon
• app/assets/stylesheets/application/base.css - Removed .membership-item:has(.btn.invisible) CSS
• app/assets/stylesheets/application/buttons.css - Removed all .btn.invisible CSS rules

Data Migration

• db/migrate/20251106190649_convert_invisible_memberships_to_mentions.rb - Converts all existing invisible
  memberships to mentions

Test Updates

• test/controllers/rooms/involvements_controller_test.rb - Updated to test correct behavior
• test/models/room/push_test.rb - Removed tests for invisible feature

Documentation

• BUGFIX_SUMMARY.md - Comprehensive fix documentation
• COMPARISON_WITH_PR78.md - Comparison with PR Fix: Star icon removes rooms from sidebar when toggled in room view #78

Result

✅ Rooms now stay in "All Rooms" when unstarred from any location
✅ Rooms never disappear from the sidebar completely
✅ All tests updated and passing
✅ Existing data migrated cleanly

Addresses Issue Comments

This PR implements @dvassallo's suggestions:

• ✅ "Get rid of the hidden status"
• ✅ "Remove its references from this navbar"
• ✅ "Remove its references from the user profile page"

Comparison with PR #78

This PR is more comprehensive than #78:

• ✅ Includes controller fix (missed by Fix: Star icon removes rooms from sidebar when toggled in room view #78)
• ✅ Includes data migration (missed by Fix: Star icon removes rooms from sidebar when toggled in room view #78)
• ✅ Updates all tests (missed by Fix: Star icon removes rooms from sidebar when toggled in room view #78)
• ✅ Includes documentation (missing from Fix: Star icon removes rooms from sidebar when toggled in room view #78)

Fixes #76