v1.8.9.7

Clean up dead code: unused source sets, classes, resources, deps

- Remove dead source sets src/conversations/, src/quicksy/, src/free/
  (not compiled - only git/playstore flavors exist)
- Remove unused classes: MonoclesViewPager, MessageTextActionModeCallback,
  SwipeDetector, utils (getThemedColor)
- Remove unused layouts: activity_calls, activity_gridview_stickers,
  activity_updater, presence_template; unused menu message_text_actions
- Remove ~44 unused drawables incl. all density variants of ic_call_*,
  ic_content_copy_*, client_* icons
- Remove unused gradle deps: nineoldandroids, dotsindicator,
  AndroidPhotoFilters, easypermissions; dedupe blurhash and tomlj
- Remove dead Spotless plugin (applied but never configured)
- Fix pre-existing build error: rounded_tag.xml referenced nonexistent
  @color/colorAccent (theme attr) -> ?attr/colorAccent
- AGENTS.md: drop stale Spotless and legacy source-set notes