feat(shopping): editable quantity field per item

Each item row now has an editable qty input showing the planned amount
(e.g. "6 × ½ kg"). Changes are auto-saved via debounce to the prices
table and highlighted in accent color when modified from the default.

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

app.py

@@ -103,7 +103,7 @@ def get_db():
 def init_db():
     with get_db() as c:
         c.execute("""CREATE TABLE IF NOT EXISTS prices (
-            key TEXT PRIMARY KEY, price REAL, checked INT DEFAULT 0, updated TEXT)""")
+            key TEXT PRIMARY KEY, price REAL, checked INT DEFAULT 0, qty TEXT, updated TEXT)""")
         c.execute("""CREATE TABLE IF NOT EXISTS extras (
             id INTEGER PRIMARY KEY AUTOINCREMENT,
             name TEXT, section_key TEXT, price REAL,
@@ -115,6 +115,10 @@ def init_db():
             c.execute("ALTER TABLE extras ADD COLUMN section_key TEXT")
         except Exception:
             pass
+        try:
+            c.execute("ALTER TABLE prices ADD COLUMN qty TEXT")
+        except Exception:
+            pass
 
 
 @app.route('/')
@@ -136,13 +140,14 @@ def save_price():
     key = d.get('key')
     price = d.get('price')
     checked = int(d.get('checked', 0))
+    qty = d.get('qty') or None
     if not key:
         return jsonify(error='missing key'), 400
     with get_db() as c:
-        c.execute("""INSERT INTO prices(key,price,checked,updated) VALUES(?,?,?,?)
+        c.execute("""INSERT INTO prices(key,price,checked,qty,updated) VALUES(?,?,?,?,?)
             ON CONFLICT(key) DO UPDATE SET price=excluded.price,
-            checked=excluded.checked, updated=excluded.updated""",
-            (key, price, checked, datetime.now().isoformat()))
+            checked=excluded.checked, qty=excluded.qty, updated=excluded.updated""",
+            (key, price, checked, qty, datetime.now().isoformat()))
     return jsonify(ok=True)