Add budget rollover: copy from previous month (#1100)

* Add budget rollover: copy from previous month

When navigating to an uninitialized budget month, show a prompt
offering to copy amounts from the most recent initialized budget.
Copies budgeted_spending, expected_income, and all matching category
allocations. Also fixes over-allocation warning showing on uninitialized
budgets.

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

* Redirect copy_previous to categories wizard for review

Matches the normal budget setup flow (edit → categories → show)
so users can review/tweak copied allocations before confirming.

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

* Address code review: eager-load categories, guard against overwrite

- Add .includes(:budget_categories) to most_recent_initialized_budget
  to avoid N+1 when copy_from! iterates source categories
- Guard copy_previous action against overwriting already-initialized
  budgets (prevents crafted POST from clobbering existing data)
- Add i18n key for already_initialized flash message

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

* Add invariant guards to copy_from! for defensive safety

Validate that source budget belongs to the same family and precedes
the target budget before copying. Protects against misuse from
other callers beyond the controller.

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

* Fix button overflow on small screens in copy previous prompt

Stack buttons vertically on mobile, side-by-side on sm+ breakpoint.

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

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

app/controllers/budgets_controller.rb

@@ -1,11 +1,12 @@
 class BudgetsController < ApplicationController
-  before_action :set_budget, only: %i[show edit update]
+  before_action :set_budget, only: %i[show edit update copy_previous]
 
   def index
     redirect_to_current_month_budget
   end
 
   def show
+    @source_budget = @budget.most_recent_initialized_budget unless @budget.initialized?
   end
 
   def edit
@@ -17,6 +18,22 @@ class BudgetsController < ApplicationController
     redirect_to budget_budget_categories_path(@budget)
   end
 
+  def copy_previous
+    if @budget.initialized?
+      redirect_to budget_path(@budget), alert: t("budgets.copy_previous.already_initialized")
+      return
+    end
+
+    source_budget = @budget.most_recent_initialized_budget
+
+    if source_budget
+      @budget.copy_from!(source_budget)
+      redirect_to budget_budget_categories_path(@budget), notice: t("budgets.copy_previous.success", source_name: source_budget.name)
+    else
+      redirect_to budget_path(@budget), alert: t("budgets.copy_previous.no_source")
+    end
+  end
+
   def picker
     render partial: "budgets/picker", locals: {
       family: Current.family,