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/templates/partials/journal_editor_toolbar.html

@@ -0,0 +1,20 @@
+<div class="markdown-toolbar">
+  <span x-show="!previewMode" style="display:flex; gap:4px; flex-wrap:wrap;">
+    <button type="button" class="md-tool-btn" title="굵게" @click="$el.closest('.field').querySelector('textarea')._easymde?.toggleBold()"><strong>B</strong></button>
+    <button type="button" class="md-tool-btn" title="기울임" @click="$el.closest('.field').querySelector('textarea')._easymde?.toggleItalic()"><em>I</em></button>
+    <button type="button" class="md-tool-btn" title="취소선" @click="$el.closest('.field').querySelector('textarea')._easymde?.toggleStrikethrough()"><s>S</s></button>
+    <button type="button" class="md-tool-btn" title="제목" @click="$el.closest('.field').querySelector('textarea')._easymde?.toggleHeadingSmaller()">H</button>
+    <button type="button" class="md-tool-btn" title="인용" @click="$el.closest('.field').querySelector('textarea')._easymde?.toggleBlockquote()">❝</button>
+    <button type="button" class="md-tool-btn" title="글머리 목록" @click="$el.closest('.field').querySelector('textarea')._easymde?.toggleUnorderedList()">•</button>
+    <button type="button" class="md-tool-btn" title="번호 목록" @click="$el.closest('.field').querySelector('textarea')._easymde?.toggleOrderedList()">1.</button>
+    <button type="button" class="md-tool-btn" title="코드" @click="$el.closest('.field').querySelector('textarea')._easymde?.toggleCodeBlock()">&lt;/&gt;</button>
+    <button type="button" class="md-tool-btn" title="링크" @click="$el.closest('.field').querySelector('textarea')._easymde?.drawLink()">🔗</button>
+  </span>
+  <button
+    type="button"
+    class="md-tool-btn md-preview-toggle"
+    style="margin-left:auto;"
+    @click="togglePreview()"
+    x-text="previewMode ? '✏️ 편집' : '👁 미리보기'"
+  ></button>
+</div>