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/css/style.css

@@ -902,8 +902,129 @@ label {
 }
 
 .journal-entry-content {
-  white-space: pre-wrap;
   margin-top: 4px;
+  line-height: 1.6;
+}
+
+.journal-preview {
+  margin-top: 6px;
+  padding: 10px 12px;
+  border: 1px dashed var(--color-border);
+  border-radius: var(--radius-control);
+  background: var(--color-bg);
+  min-height: 24px;
+  font-size: 14px;
+}
+
+/* 마크다운 에디터 툴바 (EasyMDE는 toolbar:false로 끄고 여기서 자체 버튼으로 대체 —
+   EasyMDE 기본 툴바는 Font Awesome CDN을 전제로 해서 이 앱의 "CDN 금지" 원칙과 안 맞는다) */
+.markdown-toolbar {
+  display: flex;
+  flex-wrap: wrap;
+  gap: 4px;
+  margin-bottom: 6px;
+}
+
+.md-tool-btn {
+  min-width: 30px;
+  height: 30px;
+  padding: 0 6px;
+  border: 1px solid var(--color-border);
+  border-radius: 6px;
+  background: var(--color-surface);
+  color: var(--color-text);
+  font-size: 14px;
+  cursor: pointer;
+}
+
+.md-tool-btn:active {
+  background: var(--color-bg);
+}
+
+/* EasyMDE(CodeMirror) 컨테이너를 이 앱의 입력 필드 톤에 맞춘다 */
+.markdown-editor + .EasyMDEContainer .CodeMirror {
+  border: 1px solid var(--color-border);
+  border-radius: var(--radius-control);
+  background: var(--color-bg);
+  color: var(--color-text);
+  font-family: inherit;
+  font-size: 15px;
+  padding: 6px 8px;
+}
+
+.markdown-editor + .EasyMDEContainer .CodeMirror-cursor {
+  border-left-color: var(--color-text);
+}
+
+.markdown-editor + .EasyMDEContainer .editor-statusbar {
+  color: var(--color-text-muted);
+}
+
+.journal-entry-content > *:first-child {
+  margin-top: 0;
+}
+
+.journal-entry-content > *:last-child {
+  margin-bottom: 0;
+}
+
+.journal-entry-content p,
+.journal-entry-content ul,
+.journal-entry-content ol,
+.journal-entry-content blockquote,
+.journal-entry-content pre {
+  margin: 0 0 8px;
+}
+
+.journal-entry-content h1,
+.journal-entry-content h2,
+.journal-entry-content h3,
+.journal-entry-content h4 {
+  margin: 12px 0 6px;
+  line-height: 1.3;
+}
+
+.journal-entry-content h1 { font-size: 19px; }
+.journal-entry-content h2 { font-size: 17px; }
+.journal-entry-content h3,
+.journal-entry-content h4 { font-size: 15px; }
+
+.journal-entry-content ul,
+.journal-entry-content ol {
+  padding-left: 20px;
+}
+
+.journal-entry-content blockquote {
+  margin-left: 0;
+  padding-left: 10px;
+  border-left: 3px solid var(--color-accent);
+  color: var(--color-text-muted);
+}
+
+.journal-entry-content code {
+  font-family: ui-monospace, SFMono-Regular, Menlo, Consolas, monospace;
+  font-size: 13px;
+  background: var(--color-bg);
+  border: 1px solid var(--color-border);
+  border-radius: 4px;
+  padding: 1px 5px;
+}
+
+.journal-entry-content pre {
+  background: var(--color-bg);
+  border: 1px solid var(--color-border);
+  border-radius: var(--radius-control);
+  padding: 10px;
+  overflow-x: auto;
+}
+
+.journal-entry-content pre code {
+  border: none;
+  padding: 0;
+}
+
+.journal-entry-content a {
+  color: var(--color-accent);
 }
 
 .journal-entry-tags {