refactor: phase 1 - testability improvements

- Add Config dataclass with get_config()/set_config() for injection
- Add Config.from_args() for CLI argument parsing (testable)
- Add platform font path detection (Darwin/Linux)
- Bound translate cache with @lru_cache(maxsize=500)
- Add fixtures for external dependencies (network, feeds, config)
- Add 15 tests for Config class, from_args, and platform detection

This enables testability by:
- Allowing config injection instead of global mutable state
- Supporting custom argv in from_args() for testing
- Providing reusable fixtures for mocking network/filesystem
- Preventing unbounded memory growth in translation cache

Fixes: _arg_value/_arg_int not accepting custom argv

engine/translate.py

@@ -7,26 +7,16 @@ import json
 import re
 import urllib.parse
 import urllib.request
+from functools import lru_cache
 
 from engine.sources import LOCATION_LANGS
 
-_TRANSLATE_CACHE = {}
+TRANSLATE_CACHE_SIZE = 500
 
 
-def detect_location_language(title):
-    """Detect if headline mentions a location, return target language."""
-    title_lower = title.lower()
-    for pattern, lang in LOCATION_LANGS.items():
-        if re.search(pattern, title_lower):
-            return lang
-    return None
-
-
-def translate_headline(title, target_lang):
-    """Translate headline via Google Translate API (zero dependencies)."""
-    key = (title, target_lang)
-    if key in _TRANSLATE_CACHE:
-        return _TRANSLATE_CACHE[key]
+@lru_cache(maxsize=TRANSLATE_CACHE_SIZE)
+def _translate_cached(title: str, target_lang: str) -> str:
+    """Cached translation implementation."""
     try:
         q = urllib.parse.quote(title)
         url = (
@@ -39,5 +29,18 @@ def translate_headline(title, target_lang):
         result = "".join(p[0] for p in data[0] if p[0]) or title
     except Exception:
         result = title
-    _TRANSLATE_CACHE[key] = result
     return result
+
+
+def detect_location_language(title):
+    """Detect if headline mentions a location, return target language."""
+    title_lower = title.lower()
+    for pattern, lang in LOCATION_LANGS.items():
+        if re.search(pattern, title_lower):
+            return lang
+    return None
+
+
+def translate_headline(title: str, target_lang: str) -> str:
+    """Translate headline via Google Translate API (zero dependencies)."""
+    return _translate_cached(title, target_lang)