In `@app/assets/javascripts/discourse/controllers/topic-unsubscribe.js.es6` around
lines 5 - 7, Rename the misspelled controller property stopNotificiationsText to
stopNotificationsText in topic-unsubscribe.js.es6 (preserve the
.property("model.fancyTitle") observer) and update the corresponding reference
in unsubscribe.hbs (and any other code/tests referencing stopNotificiationsText)
to the corrected stopNotificationsText so bindings and templates continue to
work.

In `@app/assets/javascripts/discourse/templates/topic/unsubscribe.hbs` around
lines 1 - 7, Rename the misspelled property stopNotificiationsText to
stopNotificationsText everywhere it’s defined and referenced: update the
template usage ({{{stopNotificiationsText}}}) and change the corresponding
property in the controller/component that provides it (where
stopNotificiationsText is declared/computed), then run tests/grep to update any
other occurrences (helpers, tests, or usages like bindings) to the corrected
stopNotificationsText to keep behavior identical but fix the typo.

In `@app/controllers/topics_controller.rb` around lines 98 - 116, In unsubscribe,
TopicUser.find_by may return nil causing a NoMethodError on tu; update the
unsubscribe action to handle a missing TopicUser: fetch with
TopicUser.find_or_initialize_by(user_id: current_user.id, topic_id:
params[:topic_id]) (or explicitly return/redirect if you prefer), then set
tu.notification_level using the same conditional logic and call tu.save!; keep
the rest of the flow (perform_show_response and redirect logic) unchanged and
reference the tu/local variable, TopicUser.find_by/find_or_initialize_by, and
the unsubscribe method.

In `@app/controllers/topics_controller.rb` around lines 107 - 111, The current GET
handler mutates TopicUser.notification_level (tu.notification_level) by toggling
between notification_levels[:regular] and [:muted]; change it so GET no longer
mutates state: have the controller render a confirmation page on GET (showing
the pending unsubscribe) and perform the actual update in a separate POST action
(e.g., unsubscribe_confirm or the same action constrained to POST). Also change
the mutation logic to be one-way: when performing the POST update, set
tu.notification_level = TopicUser.notification_levels[:muted] (do not toggle)
and persist the change; keep references to the existing tu variable and
TopicUser.notification_levels to locate the code.

In `@app/views/email/notification.html.erb` around lines 5 - 20, The
respond_instructions footer is rendered twice when context_posts.present? is
true; remove the duplicated instance inside the conditional block so only the
unconditional "<div class='footer'>%{respond_instructions}</div>" remains at the
bottom, leaving the context_posts block (and its render partial: 'email/post')
intact and unchanged.

In `@app/views/email/notification.html.erb` at line 10, The class attribute on the
H4 element is using a CSS selector dot (class='.previous-discussion') which
results in a literal dot in the HTML class and prevents selectors like
.previous-discussion from matching; edit the H4 tag in notification.html.erb
(the element with <%= t "user_notifications.previous_discussion" %>) to remove
the leading dot so the class is previous-discussion, and update any CSS if it
was accidentally written to target the dotted literal class.

In `@config/routes.rb` at line 439, The slug-less summary route (get
"t/:topic_id/summary") is missing defaults: {summary: true}, so params[:summary]
won't be set when hitting topics#show; update that route to include defaults:
{summary: true} like the slugged route (get "t/:slug/:topic_id/summary" =>
"topics#show", defaults: {summary: true}, constraints: {topic_id: /\d+/}) so
both routes set params[:summary] for TopicsController#show.

In `@config/routes.rb` around lines 440 - 441, The unsubscribe route currently
maps GET "t/:slug/:topic_id/unsubscribe" and GET "t/:topic_id/unsubscribe" to
topics#unsubscribe which mutates state (calls tu.save!) and is unsafe; change
the routes to use a non-idempotent verb (POST or DELETE) for the state-changing
endpoint and/or implement a GET confirmation flow: keep a GET action (e.g.,
topics#unsubscribe_confirm) that renders a confirmation page and move the actual
mutation into a separate POST/DELETE action (e.g., topics#unsubscribe or
topics#destroy_subscription) which performs the tu.save! logic; update the
routes to point GET -> topics#unsubscribe_confirm and POST/DELETE ->
topics#unsubscribe (or new name), and adjust the controller
(topics_controller.rb) to split the confirmation and mutation accordingly and
guard the mutation behind the non-GET verb.