feat: open Markdown files and in-app updater

Markdown:
- File > Open Markdown File... and welcome-screen 'Open .md' import
  one or more .md files as new books (named after first H1 or the
  filename; originals never modified, collision-safe folder names).
- Book dirs and .md files passed on the command line or via desktop
  file associations now open directly; single-instance lock forwards
  second launches to the running window.

Updates:
- File > Check for Updates... and welcome-screen 'Update' button:
  queries the Gitea releases API, one-click installs a newer release
  into ~/.local/share/Folio via a detached swap script that waits for
  app exit, keeps the old tree until the swap succeeds, and relaunches.
  Download is gzip-verified and restricted to the trusted release host.
  No root, no reinstall.

Tests: 19 new assertions in tests/run-update-import-test.mjs (wired
into npm test); full suite green, tsc clean.

package.json

@@ -15,7 +15,7 @@
     "dev": "concurrently -k \"npm run build:watch\" \"wait-on dist/main.js && npm run electron\"",
     "start": "npm run build && npm run electron",
     "lint": "tsc --noEmit",
-    "test": "node tests/run-project-test.mjs && node tests/run-chapters-test.mjs && node tests/run-wikilinks-test.mjs && node tests/run-office-export-test.mjs && node tests/run-migration-test.mjs && node tests/run-editor-test.mjs && node tests/run-fullbook-test.mjs && node tests/run-print-test.mjs",
+    "test": "node tests/run-project-test.mjs && node tests/run-chapters-test.mjs && node tests/run-wikilinks-test.mjs && node tests/run-office-export-test.mjs && node tests/run-migration-test.mjs && node tests/run-editor-test.mjs && node tests/run-fullbook-test.mjs && node tests/run-print-test.mjs && node tests/run-update-import-test.mjs",
     "package": "electron-builder"
   },
   "devDependencies": {