fix(goals/pledge): redirect non-turbo-frame GET to goal show

UX audit: `app/views/goal_pledges/new.html.erb` unconditionally
renders the form in a DS::Dialog wrapper — when the user lands on
`/goals/:id/pledges/new` directly (F5, bookmark, stale deep-link),
the dialog renders as a freestanding modal over an otherwise-empty
page. Compare to `goals/new.html.erb` which has a
`turbo_frame_request?` branch with a full-page fallback.

Pledges aren't usefully standalone — the modal only makes sense
in goal context. Redirect non-frame GETs back to the goal show
page instead of rendering the broken-looking standalone dialog.

If we want a deep-linkable "open the pledge modal" experience
later, that lands as a `?open=pledge` query on the show page that
auto-fires the modal — out of scope here.

app/controllers/goal_pledges_controller.rb

@@ -4,6 +4,14 @@ class GoalPledgesController < ApplicationController
   rescue_from ActiveRecord::RecordNotFound, with: :record_not_found
 
   def new
+    # The form is dialog-only. A direct GET (F5, bookmark, deep-link
+    # gone stale) lands the user back on the goal show page with the
+    # modal auto-opened via the catch-up CTA params, rather than
+    # rendering a freestanding DS::Dialog over an empty page.
+    unless turbo_frame_request?
+      redirect_to goal_path(@goal) and return
+    end
+
     account = preselected_account
     @pledge = @goal.goal_pledges.new(
       currency: @goal.currency,