feat(shared-stores): add createViewStore factory for view/filter/sort

Generic factory that eliminates ~110 LOC boilerplate per module for
view mode, sort, filters, and saved filter presets with localStorage
persistence. Includes 23 tests covering all store operations.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/shared-stores/src/test/mock-environment.ts

@@ -0,0 +1,5 @@
+/** Mock for $app/environment — always acts as browser in tests */
+export const browser = true;
+export const building = false;
+export const dev = true;
+export const version = 'test';