feat(budgets): add per-user personal budgets with strict isolation (#2891)

* feat(budgets): add per-user personal budgets with strict isolation

Families can now opt into personal budgets (toggleable via family
settings): each family member gets their own budget for a given
period instead of sharing a single family-wide budget.

- Add families.personal_budgets flag and budgets.user_id, with
  partial unique indexes so shared budgets (user_id IS NULL) and
  personal budgets (user_id IS NOT NULL) can't collide.
- Budget.find_or_bootstrap scopes lookup/creation by user when the
  family has personal_budgets enabled.
- Scope most_recent_initialized_budget (used to seed a new budget
  from the prior period) by user_id so one user's copy-forward never
  bleeds into another user's budget.
- budgets.user_id cascades on user deletion so personal budgets don't
  outlive their owner.

* feat(budgets): enforce user-specific budget ownership and cascade deletion

* feat(budgets): display user name for personal budgets in budget card on the plan section

* feat(budgets): enhance personal budgets display for admins with preview feature indication

* feat(budgets): enforce user-specific budget and category visibility for personal budgets

* feat(budgets): create budget section titles and add translations notice in preferences

* feat(budgets): let household and personal budgets coexist with sharing

Previously enabling personal_budgets made the shared household budget
unreachable. Budget.find_or_bootstrap now takes an explicit household:
flag so both can be resolved independently for the same period, with a
new household_budget_enabled family setting to opt out of the household
side and keep personal budgets only.

Adds a BudgetShare model (read_only/read_write) so a member can grant
another family member access to their personal budget, enforced via
Budget#viewable_by?/editable_by? across BudgetsController,
BudgetCategoriesController, PlansController, and the read-only API.
Preferences gains a Budget sharing card (gated on preview access like
the rest of the personal budgets UI) and an owner switcher pill (
Household / mine / shared-with-me) appears on the budget page and the
Plan hub card.

Also fixes personal budgets showing the same "actual spending" as the
household budget: actual spending/income now scope to the budget
owner's own accounts instead of the viewer's full accessible set,
via a new accounts: override on IncomeStatement.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

* feat(budgets): enhance budget switcher with icons and improved styling

* feat(budgets): remove user name display from budget card and header

* feat(budgets): remove unique index on taggable_type and taggable_id in taggings

* feat(budgets): enhance budget sharing functionality and improve UI elements

* Collapse personal budget migrations

---------

Signed-off-by: JulienGourmet <69808509+jubbakka@users.noreply.github.com>
Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>
Co-authored-by: sure-admin <sure-admin@splashblot.com>

test/models/assistant/function/get_budget_test.rb

@@ -102,6 +102,20 @@ class Assistant::Function::GetBudgetTest < ActiveSupport::TestCase
     assert_equal target, month[:period][:start_date]
   end
 
+  test "does not leak another family member's personal budget in trend months" do
+    @family.update!(personal_budgets: true)
+    prior_start = Date.current.beginning_of_month << 1
+
+    other_budget = Budget.find_or_bootstrap(@family, start_date: prior_start, user: users(:family_member))
+    other_bc = other_budget.budget_categories.find { |bc| bc.category == categories(:food_and_drink) }
+    other_bc.update!(budgeted_spending: 12345)
+
+    result = @function.call("prior_months" => 1)
+
+    assert_equal 1, result[:months].length, "another member's personal budget must not surface as a trend month"
+    assert_equal 1, result[:months_unavailable]
+  end
+
   test "raises on invalid month format" do
     assert_raises(Assistant::Error) do
       @function.call("month" => "not-a-month")