chore: remove dead modules and unused dependencies (~1,180 LOC)

Deletions, each re-verified with a fresh repo-wide grep (core, web,
scripts, docs, plugin monorepo) immediately before removal:

Modules with zero live importers:
- src/background_cache_mixin.py + src/generic_cache_mixin.py (134+150
  LOC — referenced only by each other)
- src/font_test_manager.py (134 LOC)
- src/image_utils.py (22 LOC, self-documented deprecated)
- src/layout_manager.py (408 LOC — only its own test imported it) +
  test/test_layout_manager.py
- src/common/basketball_plugin_example.py (328 LOC sample)

requirements.txt entries with zero importers in core (pre-plugin-era
manager deps): icalevents, geopy, timezonefinder, unidecode. Plus the
google-auth trio (google-auth-oauthlib, google-auth-httplib2,
google-api-python-client): their only importer is the calendar PLUGIN,
which declares all three in its own requirements.txt (verified in the
monorepo and on an installed copy) — the plugin dependency installer
owns them. Existing venvs are unaffected (removal doesn't uninstall);
fresh installs get them when calendar is installed.

Two stale references cleaned (a comment in test_pillow_compat.py, a
directory listing in HOW_TO_RUN_TESTS.md). Full suite green except the
two documented pre-existing failures (circuit_breaker mock drift, fixed
in #400; clock-simple 64x32 overflow, pre-dates this series); all core
entry modules verified importing cleanly under the emulator.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FqzC1nzTWL4kaqgMaQZFam

requirements.txt

@@ -8,16 +8,11 @@ numpy>=1.24.0  # For fast array operations in ScrollHelper (compatible with 2.x)
 
 # Timezone handling
 pytz>=2024.2,<2025.0  # Updated for latest timezone data
-timezonefinder>=6.5.0,<7.0.0  # Updated for better performance and accuracy
-geopy>=2.4.1,<3.0.0
 
 # HTTP requests
 requests>=2.33.0,<3.0.0
 
 # Google API integration
-google-auth-oauthlib>=1.2.0,<2.0.0
-google-auth-httplib2>=0.2.0,<1.0.0
-google-api-python-client>=2.147.0,<3.0.0
 
 # Font rendering
 freetype-py>=2.5.1,<3.0.0
@@ -29,10 +24,8 @@ spotipy>=2.25.2,<3.0.0
 Flask>=3.1.3,<4.0.0
 
 # Text processing
-unidecode>=1.3.8,<2.0.0
 
 # Calendar integration
-icalevents>=0.1.27,<1.0.0
 
 # WebSocket support
 python-socketio>=5.14.0,<6.0.0