Dashboard: search and filter studies by tag

- The search box matches tags as well as title and passage; "#tag" searches
  tags only
- Tag chips (with counts) replace the Tag dropdown; several can be on at
  once and a study must have all of them
- Tags on each study card are clickable filters
- "No studies match" message with a Clear filters link

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/utils/studyFilter.test.js

@@ -0,0 +1,35 @@
+import { test, expect } from 'vitest';
+import { filterStudies, tagCounts, isTagQuery } from './studyFilter.js';
+
+const index = [
+  { id: 'a', title: 'Titus Notes', chapterSummary: 'Titus 1', tags: ['hope', 'leadership'] },
+  { id: 'b', title: 'New Birth', chapterSummary: 'John 3', tags: ['grace', 'hope'] },
+  { id: 'c', title: 'Grace Alone', chapterSummary: 'Romans 3', tags: [] },
+];
+const ids = (list) => list.map((e) => e.id);
+
+test('the search box finds a study by title, passage or tag', () => {
+  expect(ids(filterStudies(index, { query: 'titus' }))).toEqual(['a']);
+  expect(ids(filterStudies(index, { query: 'romans' }))).toEqual(['c']);
+  expect(ids(filterStudies(index, { query: 'lead' }))).toEqual(['a']); // a tag
+  expect(ids(filterStudies(index, { query: 'grace' }))).toEqual(['b', 'c']); // a tag on one, a title on the other
+});
+
+test('starting with # searches tags only', () => {
+  expect(isTagQuery(' #grace')).toBe(true);
+  expect(ids(filterStudies(index, { query: '#grace' }))).toEqual(['b']); // not the study named "Grace Alone"
+  expect(ids(filterStudies(index, { query: '#HOP' }))).toEqual(['a', 'b']);
+  expect(ids(filterStudies(index, { query: '#' }))).toEqual(['a', 'b', 'c']);
+  expect(ids(filterStudies(index, { query: '#nothing' }))).toEqual([]);
+});
+
+test('chosen tags must all be present, and combine with the search', () => {
+  expect(ids(filterStudies(index, { tags: ['hope'] }))).toEqual(['a', 'b']);
+  expect(ids(filterStudies(index, { tags: ['hope', 'grace'] }))).toEqual(['b']);
+  expect(ids(filterStudies(index, { tags: ['hope'], query: 'john' }))).toEqual(['b']);
+  expect(ids(filterStudies(index, { tags: ['Hope'] }))).toEqual(['a', 'b']); // case does not matter
+});
+
+test('tags are counted, most used first', () => {
+  expect(tagCounts(index)).toEqual([{ tag: 'hope', count: 2 }, { tag: 'grace', count: 1 }, { tag: 'leadership', count: 1 }]);
+});