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/features/AdjustTypography/model/store/typographySettingsStore/typographySettingsStore.svelte.ts

@@ -297,6 +297,16 @@ export class TypographySettingsStore {
     }
 }
 
+/**
+ * Default factory storage key — used when a caller doesn't pass one.
+ */
+const DEFAULT_STORAGE_KEY = 'glyphdiff:typography';
+
+/**
+ * Storage key used by the app-wide singleton (scoped to comparison view).
+ */
+const COMPARISON_STORAGE_KEY = 'glyphdiff:comparison:typography';
+
 /**
  * Creates a typography control manager
  *
@@ -306,7 +316,7 @@ export class TypographySettingsStore {
  */
 export function createTypographySettingsStore(
     configs: ControlModel<ControlId>[],
-    storageId: string = 'glyphdiff:typography',
+    storageId: string = DEFAULT_STORAGE_KEY,
 ) {
     const storage = createPersistentStore<TypographySettings>(storageId, {
         fontSize: DEFAULT_FONT_SIZE,
@@ -322,5 +332,5 @@ export function createTypographySettingsStore(
  */
 export const typographySettingsStore = createTypographySettingsStore(
     DEFAULT_TYPOGRAPHY_CONTROLS_DATA,
-    'glyphdiff:comparison:typography',
+    COMPARISON_STORAGE_KEY,
 );

src/features/ChangeAppTheme/model/store/ThemeManager/ThemeManager.svelte.ts

@@ -30,6 +30,8 @@
 
 import { createPersistentStore } from '$shared/lib';
 
+export const STORAGE_KEY = 'glyphdiff:theme';
+
 type Theme = 'light' | 'dark';
 type ThemeSource = 'system' | 'user';
 
@@ -56,7 +58,7 @@ class ThemeManager {
     /**
      * Persistent storage for user's theme preference
      */
-    #store = createPersistentStore<Theme | null>('glyphdiff:theme', null);
+    #store = createPersistentStore<Theme | null>(STORAGE_KEY, null);
     /**
      * Bound handler for system theme change events
      */

src/features/ChangeAppTheme/model/store/ThemeManager/ThemeManager.test.ts

@@ -40,8 +40,7 @@ import { ThemeManager } from './ThemeManager.svelte';
  * - MediaQueryList listener management
  */
 
-// Storage key used by ThemeManager
-const STORAGE_KEY = 'glyphdiff:theme';
+import { STORAGE_KEY } from './ThemeManager.svelte';
 
 // Helper type for MediaQueryList event handler
 type MediaQueryListCallback = (this: MediaQueryList, ev: MediaQueryListEvent) => void;

src/features/FilterAndSortFonts/api/filters/filters.ts

@@ -8,8 +8,9 @@
  */
 
 import { api } from '$shared/api/api';
+import { API_ENDPOINTS } from '$shared/api/endpoints';
 
-const PROXY_API_URL = 'https://api.glyphdiff.com/api/v1/filters' as const;
+const PROXY_API_URL = API_ENDPOINTS.filters;
 
 /**
  * Filter metadata type from backend