replace EasyMDE journal editor with plain textarea to fix iOS Korean IME jamo split

Forcing inputStyle to contenteditable (previous commit) didn't fix it on
iPhone, confirming this is CodeMirror 5's own long-standing weakness with
CJK IME composition on iOS WebKit, not just the iPad desktop-UA detection
issue. There's no reliable fix short of dropping CodeMirror for the journal
content field, so EasyMDE is removed entirely and the textarea goes back to
a native, uncontrolled <textarea> — nothing intercepts/re-renders it during
composition, so iOS's IME just works.

The markdown toolbar buttons (bold/italic/heading/quote/lists/code/link)
now manipulate the textarea's selection directly via a small JournalEditor
JS API instead of calling EasyMDE/CodeMirror commands, and image paste
tracks its upload placeholder by a unique text marker instead of a
CodeMirror bookmark. Also drops the vendored easymde.min.js/css (no longer
referenced) and bumps the service worker CACHE_NAME so stale cached copies
of the old vendor files get evicted.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/static/service-worker.js

@@ -1,7 +1,6 @@
-const CACHE_NAME = "habit-tracker-v6";
+const CACHE_NAME = "habit-tracker-v7";
 const APP_SHELL = [
   "/static/css/style.css",
-  "/static/css/vendor/easymde.min.css",
   "/static/js/app.js",
   "/static/js/push-register.js",
   "/static/js/habit-reorder.js",
@@ -10,7 +9,6 @@ const APP_SHELL = [
   "/static/js/vendor/htmx.min.js",
   "/static/js/vendor/alpine.min.js",
   "/static/js/vendor/sortable.min.js",
-  "/static/js/vendor/easymde.min.js",
   "/static/icons/icon-192.png",
   "/static/icons/icon-512.png",
   "/static/icons/icon-apple-180.png",