fix(widget): scale the agenda widget with its size (#51)

The "Upcoming" agenda widget used Glance's default SizeMode.Single: it was
composed once at the minimum size and the launcher stretched that single
RemoteViews when enlarged, so the text stayed small-widget-sized no matter how
big the widget grew. Reported as a "font size" request (#51), but it's really a
missing size-response.

Switch to SizeMode.Exact (like MonthWidget) and read LocalSize.current to pick
one of four tiers (COMPACT/REGULAR/LARGE/XLARGE), scaling type and row metrics.
Exact over Responsive so the ~30-day LazyColumn isn't replicated per tier.

Width picks the tier, height can only lower it. Width governs how much of a
title fits on a row, so it's what should drive type size; height only decides
how many rows are visible, so a tall narrow widget shows more events rather than
bigger text. Height does act as a cap, though, or a squashed widget would keep
the large type its width earned in a sliver of space. Thresholds are spread over
the width range a phone actually produces (measured on a Pixel/Nova: a compact
widget is 222dp wide, a large one 378dp) rather than a theoretical range, so the
tiers are reachable in practice; XLARGE is reserved for tablets/foldables.

COMPACT reproduces the original constants verbatim, so an existing widget is
visually unchanged. The tier logic lives in a pure, Glance-free AgendaScale.kt
(compose.ui.unit only) and is JVM-tested: the COMPACT baseline, the width
buckets, the height cap stepping a squashed widget down, and that height never
raises the tier.

No new setting: the widget follows the size the launcher/user already chose.

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

CHANGELOG.md

@@ -7,6 +7,14 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+- The "Upcoming" agenda widget now scales its text and rows to the size you give
+  it. Previously it was laid out once for the smallest size and simply stretched
+  when enlarged, so the text stayed small no matter how big you made the widget.
+  Now a bigger widget gets bigger, more readable type and roomier rows, while the
+  default size looks exactly as before — no new setting; it follows the size you
+  already chose ([#51]).
+
 ## [2.16.0] — 2026-07-17
 
 ### Added
@@ -1038,5 +1046,6 @@ automatically, with zero telemetry and no internet permission.
 [#47]: https://codeberg.org/jlmakiola/calendula/issues/47
 [#48]: https://codeberg.org/jlmakiola/calendula/issues/48
 [#49]: https://codeberg.org/jlmakiola/calendula/issues/49
+[#51]: https://codeberg.org/jlmakiola/calendula/issues/51
 [#52]: https://codeberg.org/jlmakiola/calendula/issues/52
 [#60]: https://codeberg.org/jlmakiola/calendula/issues/60