feat: Sprint 15 — session projects, code copy button, tool card toggle

Session projects: named groups for organizing sessions. Project filter
bar with chips between search and session list. Create/rename/delete
projects, assign sessions via folder icon dropdown. Stored in
projects.json, project_id on Session model. 5 new API endpoints.

Code block copy button: every code block gets a Copy button in the
language header (or top-right for plain blocks). Clipboard API with
"Copied!" feedback.

Tool card expand/collapse: messages with 2+ tool cards get an
"Expand all / Collapse all" toggle above the card group.

13 new tests (237 total), all passing. No regressions.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

static/ui.js

@@ -374,13 +374,29 @@ function renderMessages(){
       }
       const frag=document.createDocumentFragment();
       for(const tc of cards){frag.appendChild(buildToolCard(tc));}
+      // Add expand/collapse toggle for groups with 2+ cards
+      if(cards.length>=2){
+        const toggle=document.createElement('div');
+        toggle.className='tool-cards-toggle';
+        // Collect card elements before they get moved to DOM
+        const cardEls=Array.from(frag.querySelectorAll('.tool-card'));
+        const expandBtn=document.createElement('button');
+        expandBtn.textContent='Expand all';
+        expandBtn.onclick=()=>cardEls.forEach(c=>c.classList.add('open'));
+        const collapseBtn=document.createElement('button');
+        collapseBtn.textContent='Collapse all';
+        collapseBtn.onclick=()=>cardEls.forEach(c=>c.classList.remove('open'));
+        toggle.appendChild(expandBtn);
+        toggle.appendChild(collapseBtn);
+        frag.insertBefore(toggle,frag.firstChild);
+      }
       if(insertBefore) inner.insertBefore(frag,insertBefore);
       else inner.appendChild(frag);
     }
   }
   scrollToBottom();
   // Apply syntax highlighting after DOM is built
-  requestAnimationFrame(()=>{highlightCode();renderMermaidBlocks();});
+  requestAnimationFrame(()=>{highlightCode();addCopyButtons();renderMermaidBlocks();});
   // Refresh todo panel if it's currently open
   if(typeof loadTodos==='function' && document.getElementById('panelTodos') && document.getElementById('panelTodos').classList.contains('active')){
     loadTodos();
@@ -558,6 +574,36 @@ function highlightCode(container) {
   Prism.highlightAllUnder(el);
 }
 
+function addCopyButtons(container){
+  const el=container||$('msgInner');
+  if(!el) return;
+  el.querySelectorAll('pre > code').forEach(codeEl=>{
+    const pre=codeEl.parentElement;
+    if(pre.querySelector('.code-copy-btn')) return;
+    const btn=document.createElement('button');
+    btn.className='code-copy-btn';
+    btn.textContent='Copy';
+    btn.onclick=(e)=>{
+      e.stopPropagation();
+      navigator.clipboard.writeText(codeEl.textContent).then(()=>{
+        btn.textContent='Copied!';
+        setTimeout(()=>{btn.textContent='Copy';},1500);
+      });
+    };
+    const header=pre.previousElementSibling;
+    if(header&&header.classList.contains('pre-header')){
+      header.style.display='flex';
+      header.style.justifyContent='space-between';
+      header.style.alignItems='center';
+      header.appendChild(btn);
+    }else{
+      pre.style.position='relative';
+      btn.style.cssText='position:absolute;top:6px;right:6px;';
+      pre.appendChild(btn);
+    }
+  });
+}
+
 let _mermaidLoading=false;
 let _mermaidReady=false;