refactor(web): enforce content-free Umami fields at compile time

Address the Copilot review on #26.

#1 The game_start / art_style_select payload fields were typed as bare
   `string`, so free text could still slip through despite the "content-free
   by construction" claim. Add lib/options.ts as the single source of truth
   for the selector option sets (`as const` → literal-union types), have the
   home OPTS render from those arrays, and type the analytics fields from the
   derived unions (gender/art_style/plot_style/pacing/style) plus a template
   type for `card`. Free text now fails to compile; no casts at call sites.

#2 The /play heartbeat scheduled its 30s interval unconditionally. Gate the
   effect on the same NEXT_PUBLIC_UMAMI_* env used for script injection, so
   nothing is scheduled when the tracker is off (visibility check kept — a
   hidden tab still never emits).

#3 choice_select no longer emits a -1 choice_index: skip the event when the
   index can't be resolved instead of polluting the index distribution.

Verified with tsc (exit 0) and a throwaway negative test: free text in any
of the six fields raises TS2322, valid enum/template values compile.

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

lib/analytics.ts

@@ -10,6 +10,8 @@
 // indices, counts and booleans — that is what keeps these events as
 // privacy-friendly as the cookieless page-view baseline.
 
+import type { ArtStyle, Gender, Pacing, PlotStyle } from "./options";
+
 declare global {
   interface Window {
     umami?: {
@@ -21,23 +23,24 @@ declare global {
 // Per-event payload schema. Fixing each event's allowed fields turns the RULE
 // above into a compile-time guarantee: an event simply has no slot for a prompt,
 // world/style guide or vision string, so free text can't be attached by mistake
-// (a bare `Record<string, string>` would happily accept it). Keep every field an
-// enum, index, count or boolean. `never` marks events that carry no payload.
+// (a bare `Record<string, string>` would happily accept it). Every field is a
+// literal union (shared with the selector UI via ./options), index, count or
+// boolean — never a bare `string`. `never` marks events that carry no payload.
 type AnalyticsEventData = {
   game_start:
     | {
         source: "prompt";
-        gender: string;
-        art_style: string;
-        plot_style: string;
-        pacing: string;
+        gender: Gender;
+        art_style: ArtStyle;
+        plot_style: PlotStyle;
+        pacing: Pacing;
         tts: boolean;
         has_prompt: boolean;
         has_style_ref: boolean;
       }
-    | { source: "curated"; gender: string; tts: boolean; card: string }
+    | { source: "curated"; gender: Gender; tts: boolean; card: `${"m" | "f"}${number}` }
     | { source: "custom" };
-  art_style_select: { style: string };
+  art_style_select: { style: ArtStyle };
   style_image_upload: { ok: boolean };
   scene_reached: { scene_index: number };
   choice_select: {

lib/options.ts

@@ -0,0 +1,37 @@
+// Single source of truth for the home-page selector option sets. Kept as
+// `as const` so each list also yields a literal-union type: the play-start
+// UI (app/page.tsx) renders from the arrays, and the analytics schema
+// (lib/analytics.ts) types its payload fields from the unions. That shared
+// origin is what keeps the "content-free" events honest — an event field can
+// only ever be one of these fixed labels, never free-form player text.
+
+export const GENDERS = ["男性向", "女性向"] as const;
+
+export const ART_STYLES = [
+  "自动",
+  "自定义",
+  "京阿尼细腻日常",
+  "新海诚唯美光影",
+  "Galgame CG",
+  "3D 动漫电影",
+  "赛博朋克",
+  "蒸汽波",
+  "吉卜力治愈手绘",
+  "哥特庄园",
+  "废土科幻",
+  // 以下为小众/区域性画风，留作长尾选项
+  "古典厚涂油画",
+  "极简中国水墨",
+  "浮世绘木刻",
+  "莫高窟壁画",
+  "波斯细密画",
+] as const;
+
+export const PLOT_STYLES = ["平铺直叙", "多线转折", "悬疑烧脑", "治愈日常"] as const;
+
+export const PACINGS = ["慢热细腻", "紧凑爽快"] as const;
+
+export type Gender = (typeof GENDERS)[number];
+export type ArtStyle = (typeof ART_STYLES)[number];
+export type PlotStyle = (typeof PLOT_STYLES)[number];
+export type Pacing = (typeof PACINGS)[number];