Project-aware assist, link suggestions, project-scoped RAG, semantic search tool, SSE race fix

- Writing assistant: inject project notes as context (definition-tagged first), wikilink suggestions
- Link suggestions: server-side endpoint finds unlinked term occurrences, NoteEditorView sidebar panel
- Project-scoped RAG: ChatView ProjectSelector filters semantic+keyword search to selected project
- Semantic search tool: LLM search_notes upgraded to hybrid semantic (0.40 threshold) + keyword merge
- SSE race condition fix: drain remaining events after stream loop exits in chat.py and notes.py
- RAG_AUTO_SNIPPET raised 800→4000; sidebar include uses full note body; MAX_BODY_CHARS 8000→24000
- Enter-to-submit on writing assistant instruction textareas (note and task editors)
- DiffView: equal-line collapsing with 3-line context around changes

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/views/TaskEditorView.vue

@@ -107,7 +107,7 @@ const isEditing = computed(() => taskId.value !== null);
 const renderedPreview = computed(() => renderMarkdown(body.value));
 
 // AI Assist
-const assist = useAssist(body);
+const assist = useAssist(body, taskId, projectId);
 
 const assistLabel = computed(() => {
   if (assist.isProofreading.value) return "Proofreading document...";
@@ -681,6 +681,7 @@ onUnmounted(() => window.removeEventListener("beforeunload", onBeforeUnload));
                 placeholder="What should I do with this section?"
                 class="assist-instruction"
                 rows="3"
+                @keydown.enter.exact.prevent="assist.canSubmit.value && assist.submit()"
               ></textarea>
               <div class="assist-input-actions">
                 <button class="btn-generate" @click="assist.submit()" :disabled="!assist.canSubmit.value">Generate</button>