feat(web): export interactive gallery + encrypted share file

Adds a "导出图集" action at the bottom-right of the play canvas that
snapshots the current session into localStorage and opens
/gallery#id=<id> in a new tab — the original play page keeps running
untouched. In parallel, sends the doc to /api/gallery-pack and
downloads the result as a binary .infiplot file the player can send
to a friend.

The snapshot pulls in:
  - Every visited scene's image + beat graph + recorded visit trail
  - All AI-prefetched alternate scenes (a new resolvedPrefetchesRef in
    PlayInner captures each prefetch as it resolves, so abandoned
    branches the engine already paid to generate are kept)
  - Character names + basePortraitUrl (voice base64 / styleReference
    are stripped — they aren't needed for replay)

/gallery is a no-network interactive replay:
  - Per-beat advance and per-choice navigation. Picked choices are
    highlighted; unpicked choices are clickable when an alternate was
    prefetched, greyed otherwise.
  - Stack-based navigation for stepping into branches with one-tap
    "返回主线" to collapse back to the main path.
  - Top-bar batch download for scene images (including unique
    AI-prefetched branch scenes, deduped against the main path) and
    character portraits. Fetched with a per-file AbortController + 20s
    timeout in a small concurrency pool, then clicked serially.
    Prevents one slow CDN response from stranding the busy button.
  - In-progress hint banner reminding the player to allow the
    browser's "multiple downloads" prompt.
  - F-key fullscreen with a top toolbar that auto-retracts after the
    initial glance and pops back down on cursor approach.
  - Per-scene dialogue panel (fa-clock-rotate-left, matching the
    in-game history affordance).
  - "导入分享文件" entry on the empty/error state — accepts a friend's
    .infiplot, posts to /api/gallery-unpack, renders the decrypted doc.

Share-file format (.infiplot):
  - AES-256-GCM via Web Crypto (portable to Cloudflare Workers).
  - Layout: 4-byte magic "IFPL" + 1-byte version + 12-byte nonce +
    ciphertext (includes 16-byte auth tag).
  - Key derived from GALLERY_SECRET via SHA-256.
  - GCM's auth tag gives tamper-detection for free; any flip in the
    ciphertext/nonce surfaces as "文件校验失败" — same error as wrong-key,
    so the distinction can't leak server config.
  - Stateless: server keeps no record of issued files.
  - GALLERY_SECRET unset → /api/gallery-pack returns 503, the play page
    silently skips the share-file download, local view still works.
    Rotating the secret invalidates every previously-issued file.

Retention: trimGalleryExports keeps only the 2 most recent localStorage
docs; older ones are evicted before each write so quota stays flat
regardless of how many times the player exports. Share files live on
the player's own disk — no retention concern.

Adds 'gallery_export' to the analytics event schema (scene_count only —
no free text).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.env.example

@@ -124,3 +124,14 @@ NEXT_PUBLIC_UMAMI_WEBSITE_ID=
 # domain. Comma-separated, exact match: apex ≠ www (list both), no wildcards.
 # Blank → track on all hosts. e.g. infiplot.com,www.infiplot.com
 NEXT_PUBLIC_UMAMI_DOMAINS=
+
+# ---- 7. Gallery share files (optional — leave blank to disable) ----
+# Server-side secret used to AES-256-GCM encrypt a played session into a
+# binary `.infiplot` share file the player can send to a friend. Friends drop
+# the file into /gallery; the server decrypts and renders the same interactive
+# replay. GCM's built-in auth tag also gives tamper-detection for free.
+# Blank → "导出分享文件" is hidden, only the same-browser localStorage flow
+# remains. Set to any high-entropy string ≥ 32 chars (e.g. `openssl rand -hex 32`).
+# WARNING: rotating this secret invalidates every share file ever issued
+# (decryption will fail with "文件校验失败"). Only change when you're OK with that.
+GALLERY_SECRET=