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.ts

@@ -0,0 +1,85 @@
+import { Post, SourceKind } from '../types';
+
+/**
+ * Instagram-shaped paths.
+ *
+ *   /                          the archive explorer
+ *   /<archive>/                a profile, posts tab
+ *   /<archive>/reels/          a profile, reels tab
+ *   /<archive>/saved/
+ *   /<archive>/p/<shortcode>/  a single post
+ *
+ * The older `?a=&t=&p=` query form is still parsed so existing links keep
+ * working; it is never written back.
+ */
+
+export type Tab = 'posts' | 'reels' | 'saved';
+
+const TABS: Tab[] = ['posts', 'reels', 'saved'];
+
+/**
+ * Path prefixes the app must never treat as an archive name, or a profile
+ * called "api" would shadow the backend.
+ */
+const RESERVED = new Set(['api', 'archives', 'assets', 'p', 'fonts', 'sw.js', 'manifest.webmanifest']);
+
+export interface Route {
+  archive: string | null;
+  tab: Tab;
+  /** Post shortcode, i.e. the trailing segment of a post id. */
+  post: string | null;
+}
+
+/**
+ * A post's URL slug.
+ *
+ * Sidecar posts carry a directory-scoped id (`story highlights - u - H/ABC`)
+ * so ids stay unique across sources, but only the shortcode belongs in a URL.
+ */
+export const postSlug = (post: Pick<Post, 'id'>): string => {
+  const tail = post.id.split('/').pop() ?? post.id;
+  return encodeURIComponent(tail);
+};
+
+/** Find the post a slug refers to, preferring an exact id match. */
+export const findPostBySlug = (posts: Post[], slug: string): Post | undefined => {
+  const decoded = decodeURIComponent(slug);
+  return posts.find(p => p.id === decoded)
+    ?? posts.find(p => (p.id.split('/').pop() ?? p.id) === decoded);
+};
+
+/** Which tab shows a given post, so a deep link lands on the right one. */
+export const tabForSource = (source?: SourceKind): Tab => (source === 'reels' ? 'reels' : 'posts');
+
+export const parseRoute = (pathname: string, search = ''): Route => {
+  const segments = pathname.split('/').filter(Boolean).map(decodeURIComponent);
+
+  if (segments.length && !RESERVED.has(segments[0])) {
+    const [archive, second, third] = segments;
+
+    if (second === 'p' && third) return { archive, tab: 'posts', post: third };
+    if (second && TABS.includes(second as Tab)) return { archive, tab: second as Tab, post: null };
+    return { archive, tab: 'posts', post: null };
+  }
+
+  // Legacy query form: ?a=<archive>&t=<tab>&p=<post id>
+  const params = new URLSearchParams(search);
+  const archive = params.get('a');
+  const tab = params.get('t');
+  return {
+    archive: archive || null,
+    tab: tab && TABS.includes(tab as Tab) ? (tab as Tab) : 'posts',
+    post: params.get('p'),
+  };
+};
+
+export const buildPath = ({ archive, tab, post }: Route): string => {
+  if (!archive) return '/';
+
+  const base = `/${encodeURIComponent(archive)}`;
+  // A post URL omits the tab, matching Instagram; the tab is re-derived from
+  // the post itself when the link is opened.
+  if (post) return `${base}/p/${post}/`;
+  if (tab !== 'posts') return `${base}/${tab}/`;
+  return `${base}/`;
+};