feat(post-form): client-side HEIC->JPEG photo picker (U4)

Replace /post's managed filepond field with a controlled 'photos' field
(theme forms/fields/photos) + picker logic in post-form.js: magic-byte
sniff (ISO-BMFF ftyp brands, not filename/MIME), lazy import('heic-to')
only for real HEIC (deferred 3MB chunk via ESM splitting), per-thumbnail
converting/uploading/done/error states, in-flight counter gating Submit,
and fail-closed skip on conversion failure. Converted JPEGs POST to Grav's
AJAX file-upload route into the form flash (the only path copyFiles reads),
so add-page-by-form attaches them on submit. Web-format photos pass through.

Refs R8, R9, R16, R17, AE1, AE4, KTD4.

themes/intotheeast/js/post/chunk-ZWRDP37E.js

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