chore(release): prepare v0.15.0

- set version name 0.15.0 and version code 15000
- add detailed F-Droid and GitHub release notes

Author: trevarj

fastlane/metadata/android/en-US/changelogs/15000.txt

@@ -0,0 +1,5 @@
+- Compose and read IRC formatting: bold, italic, underline, monospace, colors, reverse, and more. Formatting survives drafts, replies, and split messages.
+- New Composer Tools strip groups Emoji and formatting controls; hide either independently.
+- Set custom timestamp patterns and per-chat avatar overrides.
+- Name networks while adding them.
+- Faster, steadier history sync, transport, uploads, audio, avatars, and large formatted messages, plus keyboard and composer fixes.

gradle.properties

@@ -12,5 +12,5 @@ android.sync.suppressAgpWarnings=UNSUPPORTED_PROJECT_OPTION_USE
 android.onlyEnableUnitTestForTheTestedBuildType=false
 kotlin.code.style=official
 # Deterministic source-build defaults. Release and F-Droid metadata override these per build.
-motdVersionName=0.14.12
-motdVersionCode=14012
+motdVersionName=0.15.0
+motdVersionCode=15000