feat(nav): interactive month widget + history-retracing back

On-device review follow-ups:

- Month widget grid is now tappable. Day numbers open that day and event
  bars open the event's detail, both rooted in the month view so back
  returns to the grid. Previously only the prev/next/today header
  controls responded — the grid cells were never clickable.

- Pill/drawer view switches now build a visit history instead of
  collapsing to the default view. Back retraces the views you moved
  through (a not-yet-visited view is pushed; revisiting one collapses the
  loop back to it), down to the default, then exits. Widget launches
  still reset to their own view context.

Refs #1, #2 (reported by @devinside).

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

CHANGELOG.md

@@ -12,14 +12,18 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   Appearance) lets you pick Month, Week, Day or Agenda as the view shown each
   time you start the app, instead of always opening on Week. Thanks to
   @devinside for the suggestion ([#1]).
+- The month widget is now interactive: tap any day to open it, or tap an event to
+  open its details. Previously only the month's prev/next/today controls
+  responded. Thanks to @devinside for spotting this ([#2]).
 
 ### Fixed
-- Home-screen widgets now back out the way you came in. Opening a day or event
-  from the Agenda widget keeps you in the agenda context, and the Month widget
-  keeps you in the month context — pressing Back returns to that view instead of
-  dropping you on the week view. The app now keeps a proper view history: Back
-  steps from a drilled-in day to the view you opened it from, then to your
-  default view, then leaves the app. Thanks to @devinside for reporting ([#2]).
+- Home-screen widgets now back out the way you came in. A day or event opened
+  from the Agenda widget keeps you in the agenda context, and from the Month
+  widget in the month context — pressing Back returns there instead of dropping
+  you on the week view. More broadly, the app now keeps a real view history:
+  switching views and drilling into a day are retraced by Back one step at a
+  time, down to your default view before the app exits. Thanks to @devinside for
+  reporting ([#2]).
 
 ## [2.8.0] — 2026-06-23