feat(nav): default view setting + widget-aware back stack

Replace CalendarHost's single global view slot with a top-level view
back stack rooted at a user-configurable default view. A lateral move
(pill/drawer/widget) replaces the non-home top; a date tap drills the
day view on top; a base-level BackHandler pops one level until only the
home view remains, then the system exits.

Widgets now carry their source view (EXTRA_SOURCE_VIEW) so a launch
roots the stack in that widget's view: backing out of a day/event opened
from the agenda widget returns to Agenda, and from the month widget to
Month, instead of always landing on Week. Reminder taps keep the
separate detail-key channel and leave the base view untouched.

Add a Default view setting (Settings -> Appearance) backing the stack's
home view; defaults to Week so existing users see no change.

Resolves the two Codeberg reports from @devinside:
- #1 [FR] Option to set default view
- #2 [Bug] Widget UX improvement

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

app/src/main/res/values/strings.xml

@@ -264,6 +264,7 @@
     <string name="settings_theme_system">System</string>
     <string name="settings_theme_light">Light</string>
     <string name="settings_theme_dark">Dark</string>
+    <string name="settings_default_view">Default view</string>
     <string name="settings_dynamic_color">Dynamic colour</string>
     <string name="settings_dynamic_color_unavailable">Requires Android 12 or newer</string>
     <string name="settings_week_start">Week starts on</string>
@@ -299,7 +300,7 @@
     <string name="settings_language">App language</string>
     <string name="settings_language_auto">System default</string>
     <!-- Hub category subtitles -->
-    <string name="settings_appearance_subtitle">Theme, dynamic colour, week start</string>
+    <string name="settings_appearance_subtitle">Theme, default view, week start</string>
     <string name="settings_event_form_subtitle">Default fields for new events</string>
     <string name="settings_notifications_subtitle">Event reminders</string>
     <string name="settings_section_about">About</string>