fix: band-level windowed refine_layout + programmatic map_fields to prevent 91.5% content loss

Root cause: LLM receiving full 34k-char JRXML would regenerate from scratch
instead of modifying coordinates in-place, shrinking output to ~3k chars.

Solution (programmatic node control, not prompt engineering):

- New agent/jrxml_windower.py: decompose JRXML into header (never sent to
  LLM) + individual bands. Split bands >4000 chars at element boundaries.
  Reassemble with element count validation (>10% change = rollback).

- Rewrite refine_layout: per-band windowed LLM processing (~2-4k chars
  each). LLM cannot "reimagine" the entire report.

- Rewrite map_fields: 100% programmatic regex $F{field_N} -> real name
  replacement. Zero LLM calls, zero content loss.

- _sanitize_field_name: non-ASCII chars escaped to _uXXXX_ format for
  valid JRXML identifiers.

- Tests: 48 new unit tests (windower 28 + map_fields 20). All passing.
  Full suite 385 tests, zero regressions.

frontend/src/App.vue

@@ -8,9 +8,19 @@ import ChatMessages from './components/ChatMessages.vue'
 import ProcessSection from './components/ProcessSection.vue'
 import SummaryCard from './components/SummaryCard.vue'
 import UnifiedInput from './components/UnifiedInput.vue'
+import KbSelector from './components/KbSelector.vue'
+import KbManager from './components/KbManager.vue'
+import { useKbStore } from './stores/kb'
 
 const chat = useChatStore()
 const session = useSessionStore()
+const kb = useKbStore()
+
+function handleKbChange(kbId: string) {
+  if (session.currentId) {
+    kb.bindKbToSession(session.currentId, kbId)
+  }
+}
 
 const chatContainer = ref<HTMLElement | null>(null)
 
@@ -128,6 +138,7 @@ async function handleSend(text: string, files: File[]) {
     <Sidebar @quickAction="(text) => handleSend(text, [])" />
 
     <main class="main-area">
+      <KbSelector @change="handleKbChange" />
       <div class="chat-container" ref="chatContainer">
         <ChatMessages />
         <ProcessSection />
@@ -139,6 +150,8 @@ async function handleSend(text: string, files: File[]) {
         @send="handleSend"
       />
     </main>
+
+    <KbManager />
   </div>
 </template>