Harden JSON storage (atomic writes + crash-safe loads) + suggestions lightbulb

- storage.py: atomic save_json (temp+fsync+os.replace) and load_json that
  survives missing/corrupt files (moves corrupt aside). Closes #11/#12.
- route all data modules + app.py helpers through storage (closes #31 core);
  fixes mutable DEFAULT_FUNDS/DEFAULT_CARS return (#23).
- gunicorn --workers 1 --threads 4 to remove the write race (#13); strip
  Dockerfile template scaffolding (#40).
- base.html: 💡 suggestions link in the shared nav (shows on every page).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app.py

@@ -5,6 +5,7 @@ import pdfplumber
 import re
 from datetime import datetime, date
 import calendar as cal_module
+from storage import load_json, save_json
 from categories import CATEGORIES, CATEGORY_COLORS
 from bills import load_bills, save_bills, bills_for_month, calendar_grid, get_week_num
 from sinking import (load_funds, save_funds, FUND_COLORS, monthly_needed,
@@ -57,14 +58,10 @@ def money_filter(value):
 TAX_FILE = os.path.join(os.environ.get("DATA_DIR", os.path.join(os.path.dirname(__file__), "data")), "taxes.json")
 
 def load_taxes():
-    if os.path.exists(TAX_FILE):
-        with open(TAX_FILE) as f:
-            return json.load(f)
-    return {"rates": {"se_tax": 15.3, "federal": 22.0, "wi_state": 5.3}, "set_aside": []}
+    return load_json(TAX_FILE, {"rates": {"se_tax": 15.3, "federal": 22.0, "wi_state": 5.3}, "set_aside": []})
 
 def save_taxes(data):
-    with open(TAX_FILE, "w") as f:
-        json.dump(data, f, indent=2)
+    save_json(TAX_FILE, data)
 
 STANDARD_DEDUCTION_MFJ = 30000  # 2025 married filing jointly
 
@@ -177,27 +174,11 @@ NET_WORTH_FILE = os.path.join(DATA_DIR, "net_worth_accounts.json")
 
 
 def load_nw_accounts():
-    if os.path.exists(NET_WORTH_FILE):
-        with open(NET_WORTH_FILE) as f:
-            return json.load(f)
-    return []
+    return load_json(NET_WORTH_FILE, [])
 
 
 def save_nw_accounts(accounts):
-    with open(NET_WORTH_FILE, "w") as f:
-        json.dump(accounts, f, indent=2)
-
-
-def load_json(path, default):
-    if os.path.exists(path):
-        with open(path) as f:
-            return json.load(f)
-    return default
-
-
-def save_json(path, data):
-    with open(path, "w") as f:
-        json.dump(data, f, indent=2)
+    save_json(NET_WORTH_FILE, accounts)
 
 
 def get_transactions():