fix(admin): create-playtest form validation + layout UX

[wahyuwidharto]

Six fixes to the Create/Edit Playtest form:

• Pin the step actions bar to the bottom on all steps (flex column + margin-top:auto) so it no longer floats up on the shorter steps 2/3.
• Add client-side slug format validation (^[a-z0-9][a-z0-9-]{2,63}$, onChange) so a malformed slug is caught inline instead of round-tripping to the backend.
• Validate banner image URL onBlur (a URL is invalid mid-type).
• Move the submit-error Alert banner above the Card/stepper and give it showIcon (error icon + color).
• Make auto-approve limit mandatory when Auto-Approve is on: required asterisk + short "Must be between 1 and 100000" message on empty (out-of-bounds keeps the byte-exact server message).
• Make initial code quantity mandatory for AGS Campaign: required asterisk + inline "Must be between 1 and 50000", blocking Next instead of falling through to the server-side banner.

Adds tests for the empty auto-approve-limit and required initial-code- quantity cases. admin vitest (113) + eslint green.