refactor: extract magic constants — wave 1 (UX, API, storage)

- Use existing MULTIPLIER_S/M/L from \$entities/Font in SliderArea instead
  of inlining the 0.5/0.75/1 literals (constants already existed but were
  duplicated at the call site).
- Centralize API base URL in \$shared/api/endpoints.ts (was duplicated
  between proxyFonts and FilterAndSortFonts filters api).
- Promote every 'glyphdiff:...' localStorage key to a named module-level
  STORAGE_KEY constant. Test files now import the source constant rather
  than redeclaring it (eliminates silent-typo divergence risk).

src/shared/api/endpoints.ts

@@ -0,0 +1,19 @@
+/**
+ * Centralized backend endpoint definitions.
+ *
+ * One source of truth for the proxy API base URL — individual resource
+ * modules (proxyFonts, filters) append their own paths.
+ */
+
+export const API_BASE_URL = 'https://api.glyphdiff.com/api/v1' as const;
+
+export const API_ENDPOINTS = {
+    /**
+     * Font catalog + per-id detail + batch lookup
+     */
+    fonts: `${API_BASE_URL}/fonts`,
+    /**
+     * Filter metadata (providers, categories, subsets)
+     */
+    filters: `${API_BASE_URL}/filters`,
+} as const;