feat(reciprocity): dedicated post-gate slide + daily-cap countdown

[GraysonCAdams]

Why

Two problems with the current reciprocity gate, both surfaced by a real member report:

1. The gate opened the slide-up QueueSheet. It needed to be its own dedicated in-feed slide — a hard checkpoint you advance past via an explicit Skip or Post, not a draggable bottom sheet.
2. Hitting the daily post cap failed silently. Selecting queued clips and pressing Post just said "Posted 0" with no explanation. A member literally asked "does the 10 include videos not uploaded with credits?" — yes, the daily cap counts every clip that reached the feed, and nothing told them when they could post again.

What changed

• New QueueGateSlide.svelte — a full-screen reciprocity gate slide (NOT a BaseSheet). When you're at the credit cap with queued clips, forward scroll yields to this slide. Scroll your queued items inside it; Skip for now (soft nudge) or Post to advance. You can't scroll to the next reel or back to the previous one until you act. The manual QueueSheet stays for explicit queue access (Queue nav / ?queue=true).
• Skip is a soft nudge — it grants a short grace window (a few clips) before the gate can re-arm, rather than blocking the very next swipe.
• Daily-capped users are exempt from the gate (they can't post anyway). nextPostAvailableAt() computes when the rolling-24h window next frees a slot; it's surfaced on the watch + queue-post responses and the layout seed, so the gate stays suppressed and an explicit post attempt shows a "you can post again in 3h 20m" countdown instead of "Posted 0".
• Fixed a re-open loop: posting spends a credit, and watching the very next reel earns it straight back to the cap. Keying the gate's re-arm to activeIndex progression (not the live credit balance) stops the gate from instantly re-opening after every post.

Test plan

• npm run check — 0 errors
• npm run lint — clean
• npm test — 488 unit/API tests pass (added: nextPostAvailableAt, formatPostCooldown, queue/post daily-cap)
• npm run test:e2e (desktop + mobile-ios + mobile-android) — new e2e/reciprocity/gate-slide.spec.ts: gate engages at cap, Skip dismisses, Post publishes + stays dismissed, empty queue never gates; existing queue-interactions adjusted to a non-gated user; verified stable across 5 consecutive runs at CI worker count
• Manual: at cap with queued clips, scroll → gate slide; Skip → scroll a few then re-gate; Post → publishes + advances; with a daily cap set and hit, gate suppressed and post attempt shows the countdown