fix(widget): drive agenda range via reactive Glance state

The agenda widget loaded and sliced its data in the provideGlance preamble
and captured it as a non-reactive local. updateAll() reliably recomposes a
live Glance session but does not reliably re-run that preamble, so a range
change was redrawn against the stale slice (intermittently — only when no
session was alive did it pick up the new range). This is the same platform
limitation the month widget already works around.

Mirror that pattern: load the widest selectable window once, store the range
in per-instance Glance state (AGENDA_RANGE_KEY), read it reactively via
currentState, and slice in the composition. The settings setter writes the
state into each instance and recomposes, so a range change now reflects via
plain recomposition regardless of session lifecycle.

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

.gitea/ISSUE_TEMPLATE/feature_request.md

@@ -3,7 +3,7 @@ name: Feature request
 about: Suggest an idea or improvement
 title: ""
 labels:
-  - enhancement
+  - feat
 ---
 
 ### What would you like Calendula to do?