feat(timeline): make it a board, with the axis and the names pinned

The reader who was asked what good looks like named paimon.moe's timeline,
and the thing ours was missing is what that view never loses: on a wide
window we drew more calendar and nothing that said which day, whose game, or
which event was on screen. The date axis, the lane names and the event names
all scrolled away together — and a six-week bar starts weeks off the left
edge, so its name went with its start date and left a coloured rectangle.

So it is a pane that scrolls in both directions, with the axis stuck to the
top and every name stuck to the left. A name is sticky inside its own bar, so
it can never wander onto an event it does not belong to. The axis gained the
week the schedules are actually written in: a dated tick every Monday and a
brighter rule at each month.

The frozen label column that started this went away again — it stood on the
calendar and ate the names it was meant to keep. Lane names sit on their own
line instead, and "jump to today" sits in the board's header rather than
floating over the dates it sends you back to.

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

docs/ARCHITECTURE.md

@@ -100,7 +100,7 @@ src/
       EventRow.tsx      row + meter + caption       (F2, F3)
       Meter.tsx         the depletion meter
       Legend.tsx        what the bars and colours mean
-      Timeline.tsx      calendar lanes              (F1)
+      Timeline.tsx      the board: pinned axis, lanes, bars   (F1)
       EventDetail.tsx   detail sheet, ignore action (F9)
       ProgressControls  status, effort, note        (F12)
       Dailies.tsx       today's strip, per-game reset clocks