feat(paste): smart URL → markdown-link on URL-only pastes

- src/main/UrlTitle.js — fetch a URL, return its <title>. Pure module
  with injectable fetch for tests. Decodes named + numeric + hex entities
  (AT&amp;T, Caf&#233;, &#x2014;), strips the <title> tags, collapses
  whitespace, caps the label at 200 chars. 5s timeout via AbortController,
  2 MiB body cap to avoid OOM on big downloads, streaming reader with
  overflow cancellation. Rejects non-http(s) URLs up front.
- src/main.js — IPC url-title:fetch proxies to fetchTitle.
- src/editor/smart-paste.js — CodeMirror 6 extension that detects
  URL-only pastes (one URL, surrounded only by whitespace), inserts the
  URL immediately, then async-rewrites the insertion range to
  [Title](url) once the title arrives. Multi-line / prose pastes pass
  through untouched.
- src/editor/codemirror-setup.js — accepts smartPasteFetcher option and
  pushes the extension when provided.
- src/renderer.js — passes ipcRenderer.invoke('url-title:fetch') as
  the fetcher.
- src/preload.js — url-title:fetch added to ALLOWED_SEND_CHANNELS.
- eslint.config.js — AbortController / TextDecoder / TextEncoder added
  to globals (available in Node 20+ and Chromium).

Tests (34 new):
- tests/main/UrlTitle.test.js (24): isHttpUrl scheme filter, decodeTitle
  named/numeric/hex entities + whitespace + non-string, extractTitleFromHtml
  first match + case-insensitive + null fallback, fetchTitle success +
  long-title cap + streaming body, all failure paths (non-http,
  no-fetch, non-OK, wrong content-type, no <title>, network error,
  body over maxBytes — including streaming overflow cancellation).
- tests/smart-paste.test.js (10): URL_ONLY_RE detection (bare URL,
  path/query/fragment, whitespace padding, scheme rejection, embedded
  rejection, empty/malformed), replacement format ([T](url), brackets
  in title survive, query strings preserved).

Full suite: 71 suites, 831 tests, lint+format clean.

Amit Haridas

src/editor/codemirror-setup.js

@@ -121,6 +121,7 @@ function createEditor(parentElement, options = {}) {
     showLineNumbers = true,
     vimMode = false,
     getTabExpansion = null,
+    smartPasteFetcher = null,
   } = options;
 
   const extensions = [
@@ -155,6 +156,13 @@ function createEditor(parentElement, options = {}) {
     EditorView.lineWrapping,
   ];
 
+  // Smart-paste: a URL-only paste is intercepted and rewritten to a
+  // markdown link once the page title comes back from the main process.
+  // The host (renderer) injects the IPC-backed fetcher; null disables.
+  if (smartPasteFetcher) {
+    extensions.push(require('./smart-paste').smartPaste({ fetchTitle: smartPasteFetcher }));
+  }
+
   if (showLineNumbers) {
     extensions.push(lineNumbers());
   }