feat: spellcheck menu, typewriter mode, file associations, updater hardening

English spellcheck suggestions with Add-to-dictionary in the native context menu plus a Settings toggle. Typewriter caret-centering mode. OS file associations for .md/.txt and Open File wording. Updater: release URL surfaced, single progress listener, two-byte tarball check, installer wait guard. Search/installer test suites registered; chapter-list layout polish; ignore KDE .directory files.

electron-builder.yml

@@ -22,6 +22,25 @@ linux:
     - AppImage
     - deb
 
+# OS file associations: double-click / "Open with Folio" on Markdown and
+# plain-text files imports them as books (see folio:openPathOrImport and
+# fileArgFrom in src/main/index.ts). Without this the desktop environment
+# never offers Folio for .txt/.md files.
+fileAssociations:
+  - ext:
+      - md
+      - markdown
+    name: Folio Markdown document
+    description: Markdown document opened as a Folio book
+    role: Editor
+    mimeType: text/markdown
+  - ext:
+      - txt
+    name: Folio text document
+    description: Plain-text document opened as a Folio book
+    role: Editor
+    mimeType: text/plain
+
 # Debian package options live at the config root in electron-builder v26.
 # Setting `depends` replaces the auto-generated list, so keep its entries and
 # add what Electron links but the defaults omit — notably libasound2 (ALSA),