fix(editor): AI composer 400, slash-menu arrows, drag handle, block outline, hint

- AI (space) -> 400 DeepSeek: fdAgent.generate now uses the clean no-tools
  /api/agent/generate endpoint instead of the tool-enabled conversation engine
  (which sends tools/tool_choice schemas DeepSeek rejects). Fixes the composer,
  Ask AI, AI meeting note and summarize flows.
- Slash menu arrows: selection highlight was invisible on open (nothing marked),
  so up/down appeared dead. Call _rs() on open + after filtering, and give the
  selected item an accent background + left bar. Verified with jsdom harness:
  Down/Up move selection, Enter applies.
- Block outline when selected: suppress focus rings (outline/box-shadow) on all
  editable blocks ([data-bid], .block-content, page title).
- Drag handle (6-dot): add .block-wrapper:hover/:focus-within .block-handle so
  it shows on hover for normal blocks (only column-wrappers showed it before).
- Empty-block hint: brighter (--text-secondary, opacity .75) so the
  "Press space for AI or / for commands" text is clearly visible on empty paras.
- VERSION + app.main -> 5.1.3; CHANGELOG updated.
- 271 tests pass; page renders 200; /api/agent/generate route registered;
  JS validated (node --check + jsdom).

static/css/app.css

@@ -2243,6 +2243,10 @@ button.breadcrumb-current { max-width: 260px; }
 .block-wrapper:hover {
   background: rgba(0,0,0,0.02);
 }
+.block-wrapper:hover .block-handle,
+.block-wrapper:focus-within .block-handle {
+  opacity: 1;
+}
 
 /* 6-dot drag handle */
 .block-handle {
@@ -2332,7 +2336,8 @@ button.breadcrumb-current { max-width: 260px; }
 }
 .block-content.empty::before {
   content: attr(data-placeholder);
-  color: var(--text-dim);
+  color: var(--text-secondary);
+  opacity: .75;
   pointer-events: none;
   font-style: normal;
 }
@@ -4580,10 +4585,25 @@ button.breadcrumb-current { max-width: 260px; }
 @keyframes ai-pulse { 0%,100%{opacity:.45} 50%{opacity:1} }
 .slash-menu { flex-direction: column; }
 .slash-menu .sm-footer:hover { background: var(--bg-hover); }
-.slash-item.selected { background: var(--bg-hover); }
+.slash-item.selected { background: rgba(35,131,226,.16); box-shadow: inset 2px 0 0 var(--accent,#2383E2); }
+.slash-item.selected .slash-item-name { color: var(--text); }
 .slash-item-hint { font-size: 12px; color: var(--text-dim); opacity: .7; }
 .sm-preview { font-family: inherit; }
+.sm-preview h2 { margin: 0 0 6px; }
+.sm-preview .sm-pv-label { font-size: 11px; color: var(--text-dim); text-transform: uppercase; letter-spacing: .4px; margin: 8px 0 2px; }
 #_aiC { font-family: inherit; }
+
+/* No focus "contour" on selected/editable blocks */
+.block-content:focus,
+.block-content:focus-visible,
+[data-bid]:focus,
+[data-bid]:focus-visible,
+.page-title-input:focus { outline: none !important; box-shadow: none !important; }
+
+/* Drag handle: keep hover-visible but drop the invisible default outside gutter */
+.block-handle { opacity: .0; }
+.block-wrapper:hover .block-handle,
+.block-wrapper:focus-within .block-handle { opacity: 1; }
 .aici-result p { margin: 0 0 8px; }
 .aici-result h1,.aici-result h2,.aici-result h3,.aici-result h4 { margin: 10px 0 6px; font-weight: 700; }
 .aici-result h1{font-size:1.4em}.aici-result h2{font-size:1.25em}.aici-result h3{font-size:1.1em}.aici-result h4{font-size:1em}