feat: let the timeline stack by deadline, not only by game

Lanes keep a game's events adjacent, which is what makes the board
readable for someone playing four of them — but a reader with four games
has one queue of deadlines, and lanes scatter it: the thing ending
tonight sits three lanes below the thing ending next month, and no
amount of scrolling puts them side by side. So the stacking becomes a
choice, and the choice is remembered like the scale and the view are.

The merged mode sorts with endingSoonestFirst rather than a bare end
date, so the timeline and the list cannot mean different things by the
same words — and an unannounced end keeps its place behind every dated
one instead of pretending to a deadline.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/client/state/usePrefs.ts

@@ -4,6 +4,7 @@ import type { Region } from "../../shared/schema.ts";
 import { guessRegion } from "../../shared/time.ts";
 import type { SortMode } from "./sort.ts";
 import { KEYS, readJson, writeJson } from "./storage.ts";
+import type { TimelineGroup } from "./lanes.ts";
 import { DEFAULT_DAY_WIDTH } from "./zoom.ts";
 
 /**
@@ -72,6 +73,18 @@ export interface Prefs {
    * from an older export — or a corrupted one — land on something renderable.
    */
   timelineDayWidth: number;
+  /**
+   * How the timeline stacks its bars: a lane per game, or every game together
+   * in deadline order.
+   *
+   * Remembered for the same reason `view` and `timelineDayWidth` are — it is
+   * the reader's answer to "how do I read this?", and a board that went back to
+   * lanes on every reload would make them say it again each time.
+   *
+   * Defaults to `"game"`, which is the board every existing reader already has.
+   * A stored pref wins, so shipping this moves nobody's view.
+   */
+  timelineGroup: TimelineGroup;
   /**
    * Whether to guess which events repeat daily from what the source printed.
    * Off leaves only the ones the reader marked themselves; it never discards a
@@ -99,6 +112,7 @@ function defaults(): Prefs {
     sort: "ending",
     view: "soon",
     timelineDayWidth: DEFAULT_DAY_WIDTH,
+    timelineGroup: "game",
     detectDaily: false,
     showCompleted: true,
     showIgnored: false,