journal: real markdown editor (EasyMDE) with live preview toggle, fix default template

- Replace the plain textarea with EasyMDE (vendored locally, no CDN) for
  markdown authoring: syntax highlighting, smart list continuation, and a
  custom text-based toolbar (built-in EasyMDE toolbar icons require Font
  Awesome from a CDN, which this app doesn't use). unorderedListStyle is set
  to "-" to match the app's own template convention.
- Add a preview/edit toggle button that swaps the editor for the exact same
  server-rendered markdown (via /journal/preview) shown after saving, instead
  of always showing both.
- Fix create/edit entry routes to verify the submitted category_id actually
  belongs to the current user before inserting -- every other write path in
  this app already checked ownership; this one didn't (found while manually
  testing the new editor with a typo'd category id that happened to belong to
  someone else's category, which surfaced as an IntegrityError 500 instead of
  a clean 404-equivalent).
- Fix the default "일상" category template: bare "-" bullet lines don't parse
  as list items in the markdown renderer (they need a trailing space), and
  the content_template validator was silently stripping that trailing space
  off on every save. Backfill migration updates any category still holding
  the old, broken template text.

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

app/static/service-worker.js

@@ -1,13 +1,16 @@
-const CACHE_NAME = "habit-tracker-v5";
+const CACHE_NAME = "habit-tracker-v6";
 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",
   "/static/js/journal-category-reorder.js",
+  "/static/js/journal-editor.js",
   "/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",