chore: drop dead transactions-section preferences (#3021)

`transactions_section_controller.js` was added by #454 for the upcoming
recurring-transactions section. #771 moved recurring transactions to a
dedicated tab and removed the only mount, so the controller and its whole
persistence chain have been dead since then:
`data-controller="transactions-section"` appears nowhere in app/views or
app/components, and `transactions_section_collapsed?` had no callers outside
its own definition.

Removed:

  - app/javascript/controllers/transactions_section_controller.js
  - User#update_transactions_preferences
  - User#transactions_section_collapsed?
  - TransactionsController#update_preferences + #preferences_params
  - the `patch :update_preferences` route on the transactions collection

The only caller of /transactions/update_preferences was the dead controller
itself. No tests referenced any of it — the `update_preferences` cases in
pages_controller_test cover the dashboard's route — which is how it stayed
dead unnoticed.

No migration: users who collapsed a section before #771 keep a stale
`transactions_collapsed_sections` key in `users.preferences`, and nothing
reads it after this.

The dashboard and reports section-layout preferences are untouched.

app/javascript/controllers/transactions_section_controller.js

@@ -1,96 +0,0 @@
-import { Controller } from "@hotwired/stimulus";
-
-export default class extends Controller {
-  static targets = ["content", "chevron", "button"];
-  static values = {
-    sectionKey: String,
-    collapsed: Boolean,
-  };
-
-  connect() {
-    if (this.collapsedValue) {
-      this.collapse(false);
-    }
-  }
-
-  toggle(event) {
-    event.preventDefault();
-    if (this.collapsedValue) {
-      this.expand();
-    } else {
-      this.collapse();
-    }
-  }
-
-  handleToggleKeydown(event) {
-    // Handle Enter and Space keys for keyboard accessibility
-    if (event.key === "Enter" || event.key === " ") {
-      event.preventDefault();
-      event.stopPropagation();
-      this.toggle(event);
-    }
-  }
-
-  collapse(persist = true) {
-    this.contentTarget.classList.add("hidden");
-    this.chevronTarget.classList.add("-rotate-90");
-    this.collapsedValue = true;
-    if (this.hasButtonTarget) {
-      this.buttonTarget.setAttribute("aria-expanded", "false");
-    }
-    if (persist) {
-      this.savePreference(true);
-    }
-  }
-
-  expand() {
-    this.contentTarget.classList.remove("hidden");
-    this.chevronTarget.classList.remove("-rotate-90");
-    this.collapsedValue = false;
-    if (this.hasButtonTarget) {
-      this.buttonTarget.setAttribute("aria-expanded", "true");
-    }
-    this.savePreference(false);
-  }
-
-  async savePreference(collapsed) {
-    const preferences = {
-      collapsed_sections: {
-        [this.sectionKeyValue]: collapsed,
-      },
-    };
-
-    const csrfToken = document.querySelector('meta[name="csrf-token"]');
-    if (!csrfToken) {
-      console.error(
-        "[Transactions Section] CSRF token not found. Cannot save preferences.",
-      );
-      return;
-    }
-
-    try {
-      const response = await fetch("/transactions/update_preferences", {
-        method: "PATCH",
-        headers: {
-          "Content-Type": "application/json",
-          "X-CSRF-Token": csrfToken.content,
-        },
-        body: JSON.stringify({ preferences }),
-      });
-
-      if (!response.ok) {
-        const errorData = await response.json().catch(() => ({}));
-        console.error(
-          "[Transactions Section] Failed to save preferences:",
-          response.status,
-          errorData,
-        );
-      }
-    } catch (error) {
-      console.error(
-        "[Transactions Section] Network error saving preferences:",
-        error,
-      );
-    }
-  }
-}