feat(contacts): declare READ_CONTACTS + read contact special-dates

Codeberg #15, foundation for the contact special-dates calendars. Declares the
optional, feature-gated READ_CONTACTS permission (never requested at startup)
and adds the offline, read-only contacts reader.

- AndroidManifest: READ_CONTACTS with a comment documenting the opt-in/offline
  one-way-mirror contract.
- domain/contacts: SpecialDateType + ContactSpecialDate model and a pure
  parseContactEventDate covering full (yyyy-MM-dd), year-less (--MM-dd) and
  compact (yyyyMMdd) shapes, with Feb-29 handling via a leap anchor.
- data/contacts: ContactSpecialDatesDataSource querying ContactsContract.Data
  Event rows, split by TYPE, deduped per (contact, type); returns empty without
  the permission so sync can degrade to a stalled state. Hilt-bound.
- Unit tests for the date parser (full/year-less/compact/Feb-29/malformed).

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

app/src/main/AndroidManifest.xml

@@ -5,6 +5,15 @@
     <uses-permission android:name="android.permission.READ_CALENDAR" />
     <uses-permission android:name="android.permission.WRITE_CALENDAR" />
     <uses-permission android:name="android.permission.POST_NOTIFICATIONS" />
+    <!--
+      Optional and feature-gated: only the "Contact special dates" feature reads
+      contacts, and only after the user enables it and grants this at runtime
+      (never requested at startup). Everything stays offline — birthdays and
+      other contact dates are mirrored one-way into local calendars; contacts
+      are never written and nothing leaves the device (the app has no INTERNET
+      permission). See docs/design/contact-special-dates.md.
+    -->
+    <uses-permission android:name="android.permission.READ_CONTACTS" />
     <!--
       Lets the "Reliable delivery" setting open the direct system dialog to
       exempt Calendula from battery optimisation (so reminder broadcasts aren't