feat: add favorites feature for users

- Implemented `Favorite` model with unique user-item constraint and timestamped records
- Added `favorites:index` and `favorites:toggle` views with AJAX support for toggling favorites
- Built templates and front-end assets for the favorites page and integration with item grids
- Extended header with favorites link and dynamic counter
- Added tests covering models, views, and templates for the favorites functionality
- Updated admin to enable management of favorites via Django admin interface

static/js/favorites.js

@@ -0,0 +1,114 @@
+document.addEventListener('DOMContentLoaded', function () {
+    const favoriteForms = document.querySelectorAll('[data-favorite-form]');
+
+    const pluralizeFavorites = function (count) {
+        const value = Math.abs(Number(count) || 0);
+        const lastTwo = value % 100;
+        if (lastTwo >= 11 && lastTwo <= 14) {
+            return 'товаров';
+        }
+
+        switch (value % 10) {
+            case 1:
+                return 'товар';
+            case 2:
+            case 3:
+            case 4:
+                return 'товара';
+            default:
+                return 'товаров';
+        }
+    };
+
+    const updateFavoritesCounter = function (favoriteCount) {
+        const valueNode = document.querySelector('[data-favorites-count-value]');
+        const labelNode = document.querySelector('[data-favorites-count-label]');
+
+        if (valueNode) {
+            valueNode.textContent = String(favoriteCount);
+        }
+
+        if (labelNode) {
+            labelNode.textContent = pluralizeFavorites(favoriteCount);
+        }
+    };
+
+    const ensureEmptyFavoritesState = function () {
+        const grid = document.querySelector('[data-favorites-grid]');
+        if (!grid) {
+            return;
+        }
+
+        const cards = grid.querySelectorAll('[data-item-card]');
+        if (cards.length > 0) {
+            return;
+        }
+
+        const emptyMessage = grid.dataset.emptyMessage || 'В избранном пока нет товаров.';
+        grid.innerHTML = '<p class="item-list-empty">' + emptyMessage + '</p>';
+    };
+
+    const syncButtonState = function (button, payload) {
+        const isFavorite = Boolean(payload.is_favorite);
+        const addLabel = button.dataset.labelAdd || 'Добавить в избранное';
+        const removeLabel = button.dataset.labelRemove || 'Удалить из избранного';
+        const label = isFavorite ? removeLabel : addLabel;
+
+        button.classList.toggle('is-active', isFavorite);
+        button.setAttribute('title', label);
+        button.setAttribute('aria-label', label);
+    };
+
+    favoriteForms.forEach(function (form) {
+        form.addEventListener('submit', async function (event) {
+            event.preventDefault();
+
+            const button = form.querySelector('[data-favorite-button]');
+            if (!button || form.dataset.loading === '1') {
+                return;
+            }
+
+            form.dataset.loading = '1';
+            button.disabled = true;
+
+            try {
+                const response = await fetch(form.action, {
+                    method: 'POST',
+                    body: new FormData(form),
+                    headers: {
+                        'X-Requested-With': 'XMLHttpRequest',
+                        'Accept': 'application/json',
+                    },
+                    credentials: 'same-origin',
+                });
+
+                if (!response.ok) {
+                    throw new Error('Failed to update favorites');
+                }
+
+                const payload = await response.json();
+                const card = form.closest('[data-item-card]');
+                const favoritesPage = Boolean(document.querySelector('[data-favorites-page]'));
+
+                syncButtonState(button, payload);
+                updateFavoritesCounter(payload.favorite_count);
+
+                if (card) {
+                    card.classList.toggle('is-favorite', Boolean(payload.is_favorite));
+                }
+
+                if (favoritesPage && !payload.is_favorite && card) {
+                    card.remove();
+                    ensureEmptyFavoritesState();
+                }
+            } catch (error) {
+                console.error(error);
+                HTMLFormElement.prototype.submit.call(form);
+                return;
+            } finally {
+                form.dataset.loading = '0';
+                button.disabled = false;
+            }
+        });
+    });
+});

static/js/header-category-dropdown.js

@@ -0,0 +1,56 @@
+document.addEventListener('DOMContentLoaded', function () {
+    document.querySelectorAll('[data-category-dropdown]').forEach(function (dropdown) {
+        const trigger = dropdown.querySelector('[data-category-trigger]');
+        const label = dropdown.querySelector('[data-selected-label]');
+        const valueInput = dropdown.querySelector('[data-category-value]');
+        const menu = dropdown.querySelector('[data-category-menu]');
+        const options = dropdown.querySelectorAll('[data-category-option]');
+
+        if (!trigger || !label || !valueInput || !menu) {
+            return;
+        }
+
+        const setExpanded = function (expanded) {
+            trigger.setAttribute('aria-expanded', expanded ? 'true' : 'false');
+            dropdown.classList.toggle('is-open', expanded);
+        };
+
+        const closeDropdown = function () {
+            setExpanded(false);
+        };
+
+        const openDropdown = function () {
+            setExpanded(true);
+        };
+
+        trigger.addEventListener('click', function (event) {
+            event.preventDefault();
+            if (dropdown.classList.contains('is-open')) {
+                closeDropdown();
+                return;
+            }
+
+            openDropdown();
+        });
+
+        options.forEach(function (option) {
+            option.addEventListener('click', function () {
+                label.textContent = option.dataset.label || option.textContent.trim();
+                valueInput.value = option.dataset.value || '';
+                closeDropdown();
+            });
+        });
+
+        document.addEventListener('click', function (event) {
+            if (!dropdown.contains(event.target)) {
+                closeDropdown();
+            }
+        });
+
+        document.addEventListener('keydown', function (event) {
+            if (event.key === 'Escape') {
+                closeDropdown();
+            }
+        });
+    });
+});