feat(post-form): U5 — edit prefill + edit-mode form behaviour

The card Edit link opens /post?edit=<route>. post-form.js now (KTD4/KTD9, D1/D6/D7):
- On ?edit=, disables the form and shows 'Loading entry…' before the fetch (D1),
  so slow-connection typing can't be overwritten by the incoming prefill.
- GETs /api/v1/pages<route> (credentials:include, the gpx-manager session pattern)
  and populates every field from data.header.* / data.content: title, date
  (space→T for datetime-local), content (EasyMDE), lat, lng, city, country,
  weather select, temp, transport, featured, force_connect, published toggle.
- Sets the hidden edit_path to <route>/entry.md so cache-on-save toggles
  overwrite_mode:edit and the save writes back in place (stable URL).
- Hides the photos section and skips the ≥1-photo rule (photos untouched in M1).
- Switches chrome to 'Edit entry' / 'Save changes' (D6); reveals More options.
- On fetch failure, shows an inline banner and keeps the form disabled (D7).
- Skips draft restore in edit mode; carries edit+return on the form action for a
  future re-render (D3/D5). Rebuilt via make build-assets.

Verified in a headless browser: all fields prefill correctly incl. edit_path and
the published toggle, photos hidden, chrome correct (V6); fetch-failure banner
shown with fields left disabled (D7).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01H1FrCYNq6RXdGYbn5PFrhM

themes/intotheeast/js/src/post-form.css

@@ -4,6 +4,19 @@
  * mobile layout, and the "More options" disclosure land in U5.
  */
 
+/* Edit-mode prefill-failure banner (U5, D7): shown between the heading and the
+   first field when the entry can't be loaded for editing. */
+.post-edit-error {
+    margin: 1rem 0;
+    padding: 0.75rem 1rem;
+    border: 1px solid #E5786A;
+    border-radius: 8px;
+    background: rgba(229, 120, 106, 0.12);
+    color: #E5786A;
+    font-size: 0.9rem;
+    line-height: 1.4;
+}
+
 /* EasyMDE toolbar glyphs — replace the FontAwesome icons EasyMDE expects. */
 .editor-toolbar .mde-btn::before {
     font-style: normal;