refactor: Style transaction items for untracked accounts

This commit extends the "Untrack Account" feature by applying a special style to transaction items that belong to an untracked account.

- The transaction item template is modified to apply a "dimmed" style to transactions from untracked accounts.
- The styling follows the precedence: Account (untracked) > Category (muted) > Transaction (hidden).
- The dropdown menu for transaction items now shows "Controlled by account" if the transaction's account is untracked.

app/apps/accounts/models.py

@@ -62,6 +62,11 @@ class Account(SharedObject):
         verbose_name=_("Archived"),
         help_text=_("Archived accounts don't show up nor count towards your net worth"),
     )
+    untracked_by = models.ManyToManyField(
+        settings.AUTH_USER_MODEL,
+        blank=True,
+        related_name="untracked_accounts",
+    )
 
     objects = SharedObjectManager()
     all_objects = models.Manager()  # Unfiltered manager
@@ -75,6 +80,9 @@ class Account(SharedObject):
     def __str__(self):
         return self.name
 
+    def is_untracked_by(self, user):
+        return self.untracked_by.filter(pk=user.pk).exists()
+
     def clean(self):
         super().clean()
         if self.exchange_currency == self.currency: