Add desktop tray widget + installer wizard

- Desktop widget (Python/pystray): system tray icon showing 5h usage as
  circular progress bar with Claude starburst logo, 10-step green-to-red
  color scale, right-click menu with usage stats and configuration
- Shared cache: both widget and CLI statusline read/write the same
  /tmp/claude_usage.json — only one fetcher needs to run
- Installer wizard (install_wizard.py): interactive cross-platform setup
  with component selection, session key prompt, cron/autostart config
- OS wrappers: install.sh (Linux/macOS) and install.ps1 (Windows) find
  Python 3.9+ and launch the wizard
- README with topology diagram, usage docs, and configuration reference

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

claude_usage_widget/config.py

@@ -0,0 +1,62 @@
+"""JSON config management for Claude Usage Widget."""
+
+import json
+import os
+import sys
+
+# All config lives under claude-statusline (shared with the CLI statusline)
+if sys.platform == "win32":
+    _CONFIG_DIR = os.path.join(os.environ.get("LOCALAPPDATA", os.path.expanduser("~")), "claude-statusline")
+else:
+    _CONFIG_DIR = os.environ.get("CLAUDE_STATUSLINE_CONFIG") or os.path.join(
+        os.environ.get("XDG_CONFIG_HOME", os.path.expanduser("~/.config")), "claude-statusline"
+    )
+
+CONFIG_PATH = os.path.join(_CONFIG_DIR, "widget-config.json")
+SESSION_KEY_PATH = os.path.join(_CONFIG_DIR, "session-key")
+
+DEFAULTS = {
+    "refresh_interval": 300,  # seconds
+    "org_id": "",
+}
+
+
+def load():
+    """Load config, merging with defaults for missing keys."""
+    cfg = dict(DEFAULTS)
+    try:
+        with open(CONFIG_PATH, "r") as f:
+            cfg.update(json.load(f))
+    except (FileNotFoundError, json.JSONDecodeError):
+        pass
+    return cfg
+
+
+def save(cfg):
+    """Persist config to disk."""
+    os.makedirs(os.path.dirname(CONFIG_PATH), exist_ok=True)
+    with open(CONFIG_PATH, "w") as f:
+        json.dump(cfg, f, indent=2)
+
+
+def get_session_key():
+    """Return session key from env var or shared config file."""
+    key = os.environ.get("CLAUDE_SESSION_KEY", "").strip()
+    if key:
+        return key
+    try:
+        with open(SESSION_KEY_PATH, "r") as f:
+            key = f.read().strip()
+            if key and not key.startswith("#"):
+                return key
+    except FileNotFoundError:
+        pass
+    return ""
+
+
+def set_session_key(key):
+    """Write session key to shared config file."""
+    os.makedirs(os.path.dirname(SESSION_KEY_PATH), exist_ok=True)
+    with open(SESSION_KEY_PATH, "w") as f:
+        f.write(key.strip() + "\n")
+    os.chmod(SESSION_KEY_PATH, 0o600)