feat(account,purchases): missing endpoints and a plan field fix

[imjohnbo]

Part 7 of 8. Base: #19.

What this adds

Issue #12 named nine endpoints across the account and purchases families. Five had no CLI command. One printed the wrong value.

New account subcommands:

kit account colors
kit account set-colors <hex...>
kit account creator-profile
kit account email-stats
kit account growth-stats [--starting <date>] [--ending <date>]

set-colors validates hex format and the ten-color limit before it sends.

New purchases subcommand:

kit purchases create --file purchase.json

Bug fix

kit account read the plan from plan_name, then fell back to plan. The API returns plan_type as a string and plan as an object. The fallback therefore printed [object Object].

The command now reads plan_type. It also prints the billing interval, the subscriber limit, the trial flag, the renewal date, the time zone, and the sending addresses. A test asserts that the output never contains [object Object].

Design notes

A purchase requires eleven fields, including a products array. It therefore comes from JSON rather than from eleven flags. The command names any missing required field instead of relaying a 422.

The field check tests === undefined, not falsiness. A purchase with a zero discount is valid. A test covers this.

Review notes

Tests: 334 before, 368 after.

Closes #12

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The stack

Merge from the top down. Each part builds on the one above it.

	PR	Contents
1	#14	Client query support and shared helpers
2	#15	Sequence CRUD and sequence emails
3	#16	Posts and snippets
4	#17	Broadcast filters, account-wide stats, link clicks
5	#18	Subscriber slim listing, warnings, filter
6	#19	Tag rename, remove-by-email, bulk delete
7	#20 (this one)	Account endpoints and purchase create
8	#21	Spec coverage map and its tests

Base branch: main for part 1, the previous part's branch for the rest.
GitHub retargets each base to main as the part below it merges.