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/src/post-form.css

@@ -19,3 +19,49 @@
 .EasyMDEContainer .CodeMirror {
     min-height: 180px;
 }
+
+/* ── Photo picker (U4) — functional states; Field Notes polish in U5 ── */
+.photo-picker__list {
+    list-style: none;
+    margin: 0.5rem 0 0;
+    padding: 0;
+    display: flex;
+    flex-wrap: wrap;
+    gap: 0.5rem;
+}
+.photo-card {
+    position: relative;
+    width: 84px;
+    height: 84px;
+    border-radius: 6px;
+    overflow: hidden;
+    background: rgba(0, 0, 0, 0.06);
+    display: flex;
+    align-items: center;
+    justify-content: center;
+}
+.photo-card__thumb {
+    width: 100%;
+    height: 100%;
+    object-fit: cover;
+    display: block;
+}
+.photo-card__state {
+    position: absolute;
+    left: 0;
+    right: 0;
+    bottom: 0;
+    padding: 2px 4px;
+    font-size: 0.7rem;
+    line-height: 1.2;
+    text-align: center;
+    color: #fff;
+    background: rgba(0, 0, 0, 0.55);
+}
+.photo-card.is-done .photo-card__state { background: rgba(31, 107, 90, 0.85); }
+.photo-card.is-error {
+    background: rgba(176, 0, 32, 0.12);
+    outline: 2px solid rgba(176, 0, 32, 0.5);
+}
+.photo-card.is-error .photo-card__state { background: rgba(176, 0, 32, 0.85); }
+.photo-card.is-converting .photo-card__thumb { opacity: 0.4; }