feat(views): optionally dim or hide past events

Adds two independent display settings under Settings › Appearance, both
defaulting to the current behaviour (off) so nothing changes until a user
opts in:

  • Agenda › "Past events" (Show / Dim / Hide) — events that already ended
    today can be left as-is, faded, or dropped from the list. Hiding also
    removes any day left empty, falling back to the empty state. Re-evaluated
    each minute so rows fade/fall away as they end while the screen is open.

  • Calendar › "Dim completed events" — a separate toggle that fades finished
    events in the month and week grids, kept independent of the agenda setting.

An event counts as completed once its end is at or before now (in-progress
events are never dimmed; all-day events only after their day is fully over),
via a shared EventInstance.hasEnded(now). The grids read the cut-off through
a new LocalDimCutoff CompositionLocal (mirroring LocalShowHourLines) so only
the event chips recompose on the per-minute tick, and only while dimming is on.

Also adds an "Agenda" section header so the agenda rows stand apart in the
now-busier Appearance screen, and documents the feature in the changelog.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -5,6 +5,17 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+
+### Added
+- See past events your way. Two new settings change how events that have already
+  ended are shown. **Past events** (Settings → Appearance → Agenda) lets the
+  agenda keep them as usual, dim them, or hide them from the list entirely.
+  **Dim completed events** (Settings → Appearance) separately fades finished
+  events in the month and week views. Both are off by default, an event only
+  counts as finished once it has actually ended (events still in progress are
+  never dimmed), and the list updates on its own as the day goes on.
+
 ## [2.11.2] — 2026-06-28
 
 ### Added