feat: improve visualization of tags

Previously tags where very much focused on color coding email and less about
adding additional information. They were also visualized in different ways in
different locations.

This commit gets rid of all "Color-coding" references, aligns visualization of
the tags across the whole project and tries to improve user experience of using
tags in general.

A search box is shown in the tagging control so the user can quickly search for
a tag if they have a huge (more than 10) amount of tags.

components/email/__tests__/tag-picker.test.tsx

@@ -0,0 +1,101 @@
+import { render, screen, fireEvent, within } from '@testing-library/react';
+import { describe, it, expect, beforeEach, vi } from 'vitest';
+import { TagPicker } from '../tag-picker';
+import { useSettingsStore, type KeywordDefinition } from '@/stores/settings-store';
+
+const TAGS: KeywordDefinition[] = [
+  { id: 'work', label: 'Work', color: 'blue' },
+  { id: 'work/clients', label: 'Clients', color: 'green' },
+  { id: 'work/clients/acme', label: 'Acme', color: 'red' },
+  { id: 'personal', label: 'Personal', color: 'purple' },
+];
+
+/** Ten tags is the point at which the filter box appears. */
+const MANY_TAGS: KeywordDefinition[] = Array.from({ length: 12 }, (_, i) => ({
+  id: `tag-${i}`,
+  label: i === 0 ? 'Invoices' : `Tag ${i}`,
+  color: 'blue',
+}));
+
+describe('TagPicker', () => {
+  beforeEach(() => {
+    useSettingsStore.setState({ emailKeywords: TAGS, nestedTags: true });
+  });
+
+  it('names a nested tag by its own label, not the whole path', () => {
+    render(<TagPicker selectedIds={[]} onToggle={() => {}} />);
+
+    // The tree conveys the hierarchy, so a child needs only its own name.
+    expect(screen.getByText('Clients')).toBeInTheDocument();
+    expect(screen.getByText('Acme')).toBeInTheDocument();
+    expect(screen.queryByText('Work/Clients')).not.toBeInTheDocument();
+  });
+
+  it('indents each level below its parent', () => {
+    const { container } = render(<TagPicker selectedIds={[]} onToggle={() => {}} />);
+    const acme = screen.getByText('Acme');
+
+    // Two levels down: two nested indent wrappers between it and the list.
+    const indents = acme.closest('.ps-4')?.parentElement?.closest('.ps-4');
+    expect(indents).not.toBeNull();
+    expect(container.querySelectorAll('.ps-4').length).toBe(2);
+  });
+
+  it('marks the applied tags and reports toggles by id', () => {
+    const onToggle = vi.fn();
+    render(<TagPicker selectedIds={['work/clients']} onToggle={onToggle} />);
+
+    const row = screen.getByText('Clients').closest('button')!;
+    expect(row).toHaveAttribute('aria-checked', 'true');
+    expect(screen.getByText('Work').closest('button')).toHaveAttribute('aria-checked', 'false');
+
+    fireEvent.click(row);
+    expect(onToggle).toHaveBeenCalledWith('work/clients');
+  });
+
+  it('offers the clear-all row only while something is applied', () => {
+    const { rerender } = render(<TagPicker selectedIds={[]} onToggle={() => {}} onClearAll={() => {}} />);
+    expect(screen.queryByText('remove_tag')).not.toBeInTheDocument();
+
+    rerender(<TagPicker selectedIds={['work']} onToggle={() => {}} onClearAll={() => {}} />);
+    expect(screen.getByText('remove_tag')).toBeInTheDocument();
+  });
+
+  it('hides the filter box until the list is long enough to need one', () => {
+    render(<TagPicker selectedIds={[]} onToggle={() => {}} />);
+    expect(screen.queryByLabelText('tag_filter_placeholder')).not.toBeInTheDocument();
+
+    useSettingsStore.setState({ emailKeywords: MANY_TAGS });
+    render(<TagPicker selectedIds={[]} onToggle={() => {}} />);
+    expect(screen.getAllByLabelText('tag_filter_placeholder').length).toBeGreaterThan(0);
+  });
+
+  it('flattens to matches while filtering, and says so when there are none', () => {
+    useSettingsStore.setState({ emailKeywords: MANY_TAGS });
+    const { container } = render(<TagPicker selectedIds={[]} onToggle={() => {}} />);
+
+    fireEvent.change(screen.getByLabelText('tag_filter_placeholder'), { target: { value: 'invo' } });
+    expect(within(container).getByText('Invoices')).toBeInTheDocument();
+    expect(within(container).queryByText('Tag 5')).not.toBeInTheDocument();
+
+    fireEvent.change(screen.getByLabelText('tag_filter_placeholder'), { target: { value: 'zzz' } });
+    expect(within(container).getByText('tag_no_matches')).toBeInTheDocument();
+  });
+
+  it('matches the full path, so a child is reachable by its parent name', () => {
+    useSettingsStore.setState({ emailKeywords: [...TAGS, ...MANY_TAGS] });
+    const { container } = render(<TagPicker selectedIds={[]} onToggle={() => {}} />);
+
+    fireEvent.change(screen.getByLabelText('tag_filter_placeholder'), { target: { value: 'work/cli' } });
+    // Filtered rows are flat, so they carry the whole path.
+    expect(within(container).getByText('Work/Clients')).toBeInTheDocument();
+  });
+
+  it('lists tags flat when nesting is off', () => {
+    useSettingsStore.setState({ nestedTags: false });
+    const { container } = render(<TagPicker selectedIds={[]} onToggle={() => {}} />);
+
+    expect(container.querySelectorAll('.ps-4').length).toBe(0);
+    expect(screen.getByText('Clients')).toBeInTheDocument();
+  });
+});

components/email/__tests__/thread-list-item.test.tsx

@@ -118,6 +118,67 @@ describe('ThreadListItem tag badge', () => {
   });
 });
 
+describe('ThreadListItem multi-message thread', () => {
+  beforeEach(() => {
+    useSettingsStore.setState({
+      emailKeywords: [...DEFAULT_KEYWORDS],
+      showPreview: false,
+      mailLayout: 'split',
+    });
+    useEmailStore.setState({
+      selectedEmailIds: new Set<string>(),
+      selectedMailbox: 'inbox',
+    });
+  });
+
+  function renderThread(emails: Email[], expanded = false) {
+    const [thread] = groupEmailsByThread(emails);
+    return render(
+      <ThreadListItem
+        thread={thread}
+        isExpanded={expanded}
+        expandedEmails={expanded ? emails : undefined}
+        onToggleExpand={() => {}}
+        onEmailSelect={() => {}}
+      />,
+    );
+  }
+
+  it('carries the tags of every message, not just the first', () => {
+    // A collapsed row stands in for the whole thread, so a tag applied only to
+    // a later message still has to surface.
+    renderThread([
+      makeEmail({ id: 'e1', threadId: 't1', keywords: { '$label:red': true } }),
+      makeEmail({ id: 'e2', threadId: 't1', keywords: { '$label:blue': true } }),
+    ]);
+
+    expect(screen.getByText('Red')).toBeInTheDocument();
+    expect(screen.getByText('Blue')).toBeInTheDocument();
+  });
+
+  it('names a tag shared by several messages once', () => {
+    renderThread([
+      makeEmail({ id: 'e1', threadId: 't1', keywords: { '$label:red': true } }),
+      makeEmail({ id: 'e2', threadId: 't1', keywords: { '$label:red': true } }),
+    ]);
+
+    expect(screen.getAllByText('Red')).toHaveLength(1);
+  });
+
+  it('shows each message its own tags once the thread is expanded', () => {
+    renderThread(
+      [
+        makeEmail({ id: 'e1', threadId: 't1', keywords: { '$label:red': true } }),
+        makeEmail({ id: 'e2', threadId: 't1', keywords: { '$label:blue': true } }),
+      ],
+      true,
+    );
+
+    // Once on the header and once on the message that carries it.
+    expect(screen.getAllByText('Red').length).toBeGreaterThan(1);
+  });
+});
+
 describe('ThreadListItem row content', () => {
   beforeEach(() => {
     useSettingsStore.setState({