feat: Instagram-shaped URLs, Instagram-shaped gestures, iOS-feel animations

Navigation gestures
Horizontal swipe used to advance the carousel and then, on the last slide,
fling you into the next post — one gesture meaning two things. Horizontal is
now carousel-only. On touch, vertical swipe moves between posts (down on the
first post still dismisses, keeping drag-to-close where it can't mean
"previous"). Desktop keeps the arrows outside the modal.

URLs
Permalinks now mirror Instagram:

  /<archive>/                 profile
  /<archive>/reels/           tab
  /<archive>/p/<shortcode>/   post

A post URL carries no tab, as on Instagram; the tab is re-derived from the
post's source, so opening a reel link lands on the Reels tab with next/prev
paging through reels. Sidecar posts keep directory-scoped ids internally but
expose only the shortcode. The old ?a=&t=&p= form is still parsed so existing
links keep working, and reserved prefixes (api, archives, assets…) can never be
mistaken for a profile name.

Animations
Adds a shared motion vocabulary tuned to feel native: critically damped springs
rather than fixed-duration easing, and gestures hand their exit velocity to the
animation so a flick continues instead of restarting. Post transitions animate
along the axis the input implies — vertical for a swipe, horizontal for the
arrows. Modal and story viewer present/dismiss with a scale, tiles and
highlight circles get touch-down feedback, and prefers-reduced-motion is
honoured throughout.

Adds 22 routing tests (58 total).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011uBWhwV3wFQ5MBCcMHHem7

src/lib/routing.test.ts

@@ -0,0 +1,114 @@
+import { describe, expect, it } from 'vitest';
+import { buildPath, findPostBySlug, parseRoute, postSlug, tabForSource } from './routing';
+import { Post } from '../types';
+
+const post = (id: string, source?: Post['source']): Post => ({
+  id, date: '2024-01-01', username: 'u', caption: '', media: [], thumbnail: '', source,
+});
+
+describe('parseRoute', () => {
+  it('reads the explorer root', () => {
+    expect(parseRoute('/')).toEqual({ archive: null, tab: 'posts', post: null });
+  });
+
+  it('reads a profile', () => {
+    expect(parseRoute('/4utumn07/')).toEqual({ archive: '4utumn07', tab: 'posts', post: null });
+  });
+
+  it('reads a profile without a trailing slash', () => {
+    expect(parseRoute('/4utumn07')).toEqual({ archive: '4utumn07', tab: 'posts', post: null });
+  });
+
+  it('reads a tab', () => {
+    expect(parseRoute('/4utumn07/reels/').tab).toBe('reels');
+    expect(parseRoute('/4utumn07/saved/').tab).toBe('saved');
+  });
+
+  it('reads a post in Instagram form', () => {
+    expect(parseRoute('/4utumn07/p/Db5tIoRCcvm/')).toEqual({
+      archive: '4utumn07', tab: 'posts', post: 'Db5tIoRCcvm',
+    });
+  });
+
+  it('decodes archive names containing spaces', () => {
+    expect(parseRoute('/Heejin_Bubble%20heejinmedia/').archive).toBe('Heejin_Bubble heejinmedia');
+  });
+
+  it('does not treat reserved prefixes as archives', () => {
+    for (const path of ['/api/archives', '/archives/x/y.jpg', '/assets/index.js']) {
+      expect(parseRoute(path).archive).toBeNull();
+    }
+  });
+
+  it('still understands the legacy query form', () => {
+    expect(parseRoute('/', '?a=4utumn07&t=reels&p=ABC')).toEqual({
+      archive: '4utumn07', tab: 'reels', post: 'ABC',
+    });
+  });
+
+  it('ignores an unknown tab', () => {
+    expect(parseRoute('/', '?a=u&t=bogus').tab).toBe('posts');
+  });
+});
+
+describe('buildPath', () => {
+  it.each([
+    [{ archive: null, tab: 'posts', post: null }, '/'],
+    [{ archive: '4utumn07', tab: 'posts', post: null }, '/4utumn07/'],
+    [{ archive: '4utumn07', tab: 'reels', post: null }, '/4utumn07/reels/'],
+    [{ archive: '4utumn07', tab: 'posts', post: 'Db5tIoRCcvm' }, '/4utumn07/p/Db5tIoRCcvm/'],
+  ] as const)('builds %j', (route, expected) => {
+    expect(buildPath(route as any)).toBe(expected);
+  });
+
+  it('omits the tab from a post URL, matching Instagram', () => {
+    expect(buildPath({ archive: 'u', tab: 'reels', post: 'ABC' })).toBe('/u/p/ABC/');
+  });
+
+  it('encodes archive names with spaces', () => {
+    expect(buildPath({ archive: 'a b', tab: 'posts', post: null })).toBe('/a%20b/');
+  });
+
+  it('round-trips through parseRoute', () => {
+    for (const route of [
+      { archive: '4utumn07', tab: 'posts' as const, post: null },
+      { archive: '4utumn07', tab: 'reels' as const, post: null },
+      { archive: 'Heejin_Bubble heejinmedia', tab: 'posts' as const, post: null },
+    ]) {
+      expect(parseRoute(buildPath(route))).toEqual(route);
+    }
+  });
+});
+
+describe('postSlug / findPostBySlug', () => {
+  it('uses the bare shortcode for base posts', () => {
+    expect(postSlug(post('Db5tIoRCcvm'))).toBe('Db5tIoRCcvm');
+  });
+
+  it('strips the sidecar directory from the slug', () => {
+    expect(postSlug(post('story highlights - u - Sunstory/C5dQPEYpd9W'))).toBe('C5dQPEYpd9W');
+  });
+
+  it('resolves a slug back to its post', () => {
+    const posts = [post('AAA'), post('4utumn07 - reels/BBB', 'reels')];
+    expect(findPostBySlug(posts, 'BBB')?.id).toBe('4utumn07 - reels/BBB');
+    expect(findPostBySlug(posts, 'AAA')?.id).toBe('AAA');
+  });
+
+  it('prefers an exact id match over a shortcode match', () => {
+    const posts = [post('x/ABC'), post('ABC')];
+    expect(findPostBySlug(posts, 'ABC')?.id).toBe('ABC');
+  });
+
+  it('returns undefined for an unknown slug', () => {
+    expect(findPostBySlug([post('AAA')], 'ZZZ')).toBeUndefined();
+  });
+});
+
+describe('tabForSource', () => {
+  it('sends reels to the reels tab and everything else to posts', () => {
+    expect(tabForSource('reels')).toBe('reels');
+    expect(tabForSource('posts')).toBe('posts');
+    expect(tabForSource(undefined)).toBe('posts');
+  });
+});