fix(webui,gui): repair dead export button + wire chat clear; prune UI redundancy

WebUI (internal/gateway/ui):
- BUG: the export modal's custom-range button called
  downloadStatsCsvFromForm() which was never defined — clicking it threw a
  ReferenceError and nothing downloaded. Implement it: reads #exp-from /
  #exp-to date inputs and forwards to downloadStatsCsv.
- BUG-adjacent: clearChat() existed but was reachable from no control —
  add a Clear button to the chat composer so conversation reset is actually
  possible (+ cClear i18n zh/en).
- remove byte-identical duplicate html[data-theme=dark] CSS block (15 lines)
- remove 8 dead CSS rules (.keys-grid .m-model-row .scope-add/.scope-box/
  .scope-chips .scr-blocks .tag-warn .twrap) and the never-consumed
  --accent custom property
- remove 3 dead JS functions (activeTab/findSlots/scopeUncomb; lastTab decl kept)
- remove 24 dead i18n keys x zh/en (~55 lines) — legacy of the replaced
  key-scope editor, matching the removed .scope-* styles

GUI (cmd/gui/main.js):
- BUG: stopCore() set app.isQuitting=true and nothing reset it — after using
  tray 'stop core', closing the window quit the whole app instead of hiding
  to tray, and core crash auto-restart stayed disabled. isQuitting now only
  flips in restartCore (scoped) and before-quit.

Verified: go vet/test green; node --check on all three GUI js files and the
WebUI inline script.

cmd/gui/main.js

@@ -263,7 +263,6 @@ function waitForCoreReady(timeoutMs = 10000) {
 let coreLastExit = 0;
 
 function stopCore() {
-  app.isQuitting = true;
   coreReady = false;
   if (coreProc && coreProc.exitCode === null) {
     try {