feat(sidebar): custom room sections via u.* tags (#108)

Room context menu → "Add to Section" submenu: every u.<name> tag in use
across your rooms as a checkable item, plus a "New section…" field
(validated: non-empty, ≤ 40 chars, no dots, unique). Toggling writes or
deletes the standard u.<name> room tag (order 0.5), so sections sync
across devices and other clients see the same tags. The menu row reads
"Sections: Raids, Off-topic" once a room is in any.

Home renders each section as a collapsible category between Favorites
and Rooms (alphabetical; members by tag order then name; the same
closed-state store and unread-only-when-collapsed behaviour as the built-in
categories). A sectioned room leaves the plain Rooms list but keeps a
Favorite / Low Priority placement. Empty sections don't exist by
construction; rename is retag (v2). Derivation in utils/roomSections.ts
with unit tests.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/utils/roomSections.test.ts

@@ -0,0 +1,45 @@
+import { describe, it } from 'node:test';
+import assert from 'node:assert/strict';
+import { deriveRoomSections, sectionTag, validateSectionName } from './roomSections';
+
+describe('deriveRoomSections', () => {
+  it('groups by u.* tags, orders members by order then name, sections by name', () => {
+    const { sections, sectioned } = deriveRoomSections([
+      { roomId: '!a', name: 'Alpha', tags: { 'u.Raids': { order: 0.7 } } },
+      { roomId: '!b', name: 'Beta', tags: { 'u.Raids': { order: 0.2 }, 'm.favourite': {} } },
+      { roomId: '!c', name: 'Gamma', tags: { 'u.Raids': {}, 'u.off-topic': { order: '0.1' } } },
+      { roomId: '!d', name: 'Delta', tags: { 'm.lowpriority': {} } },
+      { roomId: '!e', name: 'Eps', tags: undefined },
+    ]);
+    assert.deepEqual(
+      sections.map((s) => [s.name, s.tag, s.rooms]),
+      [
+        ['off-topic', 'u.off-topic', ['!c']],
+        ['Raids', 'u.Raids', ['!b', '!a', '!c']],
+      ],
+    );
+    assert.deepEqual([...sectioned].sort(), ['!a', '!b', '!c']);
+  });
+
+  it('has no empty sections', () => {
+    assert.deepEqual(deriveRoomSections([{ roomId: '!a', name: 'A', tags: {} }]).sections, []);
+  });
+
+  it('ignores m.* and malformed u. tags', () => {
+    const { sections } = deriveRoomSections([
+      { roomId: '!a', name: 'A', tags: { 'm.favourite': {}, 'u.': {}, 'x.y': {} } },
+    ]);
+    assert.deepEqual(sections, []);
+  });
+});
+
+describe('section names', () => {
+  it('builds the tag and validates input', () => {
+    assert.equal(sectionTag('  Raids '), 'u.Raids');
+    assert.equal(validateSectionName('Raids', []), undefined);
+    assert.ok(validateSectionName('', []));
+    assert.ok(validateSectionName('raids', ['Raids']));
+    assert.ok(validateSectionName('a.b', []));
+    assert.ok(validateSectionName('x'.repeat(41), []));
+  });
+});