* fix(merchants): preserve user-selected merchant colors on save (#2760)
Fixes#2760.
## Problem
FamilyMerchant#set_default_color callback ran on before_validation and unconditionally executed self.color = COLORS.sample.
As a result:
- Any user-selected color or color passed via API/import was discarded and replaced with a random palette color.
- Renaming or updating any merchant field re-assigned a new random color on every save.
## Fix
Guard set_default_color so it only assigns a random palette color when color is blank (self.color = COLORS.sample if color.blank?).
## Test
- Added test/models/family_merchant_test.rb testing color preservation on creation and update, as well as default color fallback when blank.
- Updated test/controllers/family_merchants_controller_test.rb to assert persisted color on create and update.
- Verified in dev container: 9 runs, 22 assertions, 0 failures, 0 errors.
- RuboCop: 0 offenses.
* fix(merchants): validate hex format and fallback to default color if invalid
Add hex format validation (/\A#[0-9A-Fa-f]{6}\z/) to FamilyMerchant#color (matching Category and Tag models) and ensure set_default_color replaces invalid hex values (e.g. from API/CSV imports) with a default palette color before saving.
Responds to Codex review feedback on PR #2802.
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Co-authored-by: erkdgn <erkdgn@users.noreply.github.com>
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