Add taken_at to readouts and default unit to quantities

Readouts gain a taken_at timestamp (distinct from created_at) that records
when the measurement was actually taken. Measurements are now ordered by
taken_at descending.

Quantities gain an optional default_unit association. When set, the unit
is pre-selected in the measurement form. A "Set as default" button on the
unit selector lets users update the default directly from the form.

- Migrations: add taken_at (datetime) to readouts,
              add default_unit_id (fk → units) to quantities
- Readout: expose taken_at in ATTRIBUTES permit-list
- Quantity: add default_unit belongs_to, expose in ATTRIBUTES
- QuantitiesController: load @user_units for form actions
- Quantities views: add Default unit column and select to form
- Readouts form: pre-select default unit; add "Set as default" button
  (readoutUnitChanged / setDefaultUnit wired up in a later commit)
- Measurements form: default taken_at input to current time
- ApplicationHelper: propagate :form option to html_options in builder
- config/environments/test.rb: allow Capybara's dynamic host
- Tests: system tests for default-unit UI on the Quantities page

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

CLAUDE.md

@@ -0,0 +1,84 @@
+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Project Overview
+
+Fixin.me is a "quantified self" Rails 7.2.3 application for personal data tracking. Users define hierarchical **quantities** (metrics to track), **units** (with optional conversion hierarchies), and **readouts** (individual measurements). There is also a non-persistent **measurement** model used as a form wrapper.
+
+## Setup
+
+Configuration files are distributed as `.dist` templates — copy and customize before use:
+
+```bash
+cp config/application.rb.dist config/application.rb
+cp config/database.yml.dist config/database.yml
+cp config/puma.rb.dist config/puma.rb
+```
+
+```bash
+bundle config --local frozen true
+bundle config --local path .gem
+bundle config --local with mysql development test   # or: pg, sqlite
+bundle install
+RAILS_ENV=development bundle exec rails db:create db:migrate db:seed
+```
+
+## Common Commands
+
+```bash
+bundle exec rails s                                          # start server
+bundle exec rails test                                       # all unit/model/controller tests
+bundle exec rails test:system                                # all system tests (Capybara + Selenium)
+bundle exec rails test test/system/units_test.rb            # single test file
+bundle exec rails test --seed 64690 --name test_add_unit    # single test by name
+bundle exec rails db:seed:export                             # export default settings as seed file
+```
+
+## Architecture
+
+### Data Model
+
+- **Quantity** — hierarchical tree (self-referential `parent_id`). Cached `depth` and `pathname` fields are recomputed via recursive CTEs on write. Direct assignment to cached fields is blocked.
+- **Unit** — optional hierarchy via `base_id` and `multiplier` for unit conversion. Multiplier precision/scale is validated by a custom validator.
+- **Readout** — single measurement: `value` (IEEE 754 float), `quantity`, `unit`, `category`.
+- **Measurement** — `ActiveModel::Model` form wrapper (not database-backed); bridges the readout creation form.
+- **User** — Devise-managed with a status enum: `admin`, `active`, `restricted`, `locked`, `disabled`. Admins can disguise as other users.
+
+### Hierarchical Queries
+
+Both `Quantity` and `Unit` use recursive CTEs for tree traversal (ordered traversal, ancestors, progenies, common ancestors). `lib/core_ext/arel/` patches Arel to support CTE with `UPDATE`/`DELETE` statements, working around Rails issue #54658.
+
+### Custom Extensions (`lib/core_ext/`)
+
+- **arel/** — CTE support for UPDATE/DELETE
+- **active_model/** — precision/scale validator used by `Unit#multiplier`
+- **active_record/** — `attr_cached` mechanism (see `ApplicationRecord`)
+- **action_view/** — record identifier suffixes
+- Miscellaneous: `Array#delete_bang`, `BigDecimal` scientific notation
+
+### Response Handling
+
+Controllers respond to both HTML and Turbo Stream formats. Errors during Turbo Stream requests trigger a redirect with flash rather than rendering inline, handled in `ApplicationController`.
+
+### Numeric Precision
+
+Readout values are stored as IEEE 754 double-precision floats (not fixed-point decimals). Rationale in `DESIGN.md`: biological values span many orders of magnitude; 15-digit float precision is sufficient and avoids conversion overhead.
+
+### Routes
+
+```
+measurements   GET/POST /measurements
+readouts       GET/POST /readouts, DELETE /readouts/:id/discard
+quantities     CRUD + POST /quantities/:id/reparent
+units          CRUD + POST /units/:id/rebase
+users          CRUD + POST /users/:id/disguise, POST /users/revert
+default/       namespace for default units import/export and admin panel
+root           → /units (authenticated), /sign_in (unauthenticated)
+```
+
+## Database Requirements
+
+The database must support:
+- Recursive CTEs with `UPDATE`/`DELETE` (MySQL ≥ 8.0, PostgreSQL, or SQLite3)
+- Decimal precision of 30+ digits