feat(timeline): let the reader set the scale

One density cannot answer both questions the board is asked. A patch cycle is
six weeks, a login campaign runs for months, and 13px a day is a compromise
between "what am I in the middle of this week?" and "how do the next three
months line up?" that serves neither well.

A pair of controls steps through a ladder of day widths and the choice is
remembered, on the same argument as the view tabs: a reader who has said how
they want to read this should not say it again on the next load.

Two things it had to get right. Zooming holds the middle of the view still —
rescaling around the left edge of a three-month board throws away whatever
they had scrolled to, and re-opening at today would undo the scrolling that
got them there. And the dated ticks thin out as the scale shrinks, because a
week is 42px at the widest setting and the dates would sit on top of each
other; the gridlines stay weekly, since they carry the rhythm rather than the
reading.

The scale is stored as the measurement, not a step number, and read through
`snapDayWidth`. An export written against a different ladder then opens on
something close to what its reader chose, and a corrupt value opens on the
default rather than a board one pixel wide.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

AGENTS.md

@@ -112,9 +112,10 @@ src/client/       React app, service worker, manifest
   state/          progress, daily log, ignores, prefs, sort — all localStorage
                   useCustom.ts — the reader's own games and events (PRD F13)
                   lens.ts — who sees which rows (focus, outstanding, next-to-expire); pure
+                  zoom.ts — the timeline's scale ladder; pure
 scripts/          build-feed.ts, build-static.ts, parse-fixture.ts (offline), refresh-sources.ts (fetches)
 serve.ts          static server + /api/health
-test/             560 tests
+test/             579 tests
 fixtures/<game>/  raw HTML + .expected.json per source — pinned, kept forever
 snapshots/        current page per source, rewritten by refresh — see its README
 ```