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/motion.ts

@@ -0,0 +1,44 @@
+import type { Transition } from 'motion/react';
+
+/**
+ * Shared motion vocabulary, tuned to feel like a native iOS app.
+ *
+ * Two rules do most of the work:
+ *  - UIKit animates with springs, not fixed-duration easing, so gestures hand
+ *    their exit velocity to the animation and motion continues rather than
+ *    restarting.
+ *  - iOS springs are critically damped. They settle firmly with no visible
+ *    bounce; overshoot reads as "web animation", not "native".
+ */
+
+/** The curve UIKit uses for sheet presentation. */
+export const IOS_EASE = [0.32, 0.72, 0, 1] as const;
+
+/** Moving between peers: carousel slides, next/previous post. */
+export const NAVIGATE: Transition = { type: 'spring', stiffness: 420, damping: 40, mass: 1 };
+
+/** Presenting or dismissing a surface. Slightly softer than navigation. */
+export const PRESENT: Transition = { type: 'spring', stiffness: 320, damping: 34, mass: 1 };
+
+/** Backdrops and cross-fades, where a spring would feel fussy. */
+export const FADE: Transition = { duration: 0.28, ease: IOS_EASE };
+
+/** Touch-down feedback. Fast enough to feel like a direct response. */
+export const PRESS: Transition = { type: 'spring', stiffness: 600, damping: 30 };
+
+/**
+ * Continue a drag into its animation.
+ *
+ * Handing the gesture's exit velocity to the spring is what separates "the
+ * sheet kept moving because I flicked it" from "the sheet started a new
+ * animation once I let go".
+ */
+export const withVelocity = (velocity: number, base: Transition = NAVIGATE): Transition => ({
+  ...base,
+  velocity,
+});
+
+/** True when the viewer has asked the OS to reduce motion. */
+export const prefersReducedMotion = () =>
+  typeof window !== 'undefined' &&
+  window.matchMedia('(prefers-reduced-motion: reduce)').matches;