feat: view-only mode (Issue #35)

A 👁 button below the notebooks hides the tools. Pen, finger and mouse
then only scroll and zoom (the pen glides on like a finger), editing
shortcuts are ignored, Page Down no longer appends pages, and selection,
text box and ruler are closed when entering the mode.

The toolbar size estimate now includes the measured frame overhead and
can shrink buttons to 26 px, so the extra button still fits at 768 px.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/i18n/en.rs

@@ -211,6 +211,9 @@ pub const EN: &[(&str, &str)] = &[
     ("Notizbuch", "Notebook"),
     ("Notizbücher verwalten (aktuell: {title})", "Manage notebooks (current: {title})"),
     ("Nur Bilder (PNG, JPEG, GIF, WebP, BMP) und PDFs können eingefügt werden", "Only images (PNG, JPEG, GIF, WebP, BMP) and PDFs can be inserted"),
+    ("Nur ansehen – tippen, um wieder zu schreiben", "View only – tap to write again"),
+    ("Nur ansehen – 👁 erneut tippen, um zu schreiben", "View only – tap 👁 again to write"),
+    ("Nur ansehen: Stift, Finger und Maus verschieben nur das Blatt, nichts wird verändert", "View only: pen, finger and mouse just move the page, nothing gets changed"),
     ("Nur Textmarker radieren – Handschrift darunter bleibt", "Erase highlighter only – handwriting underneath stays"),
     ("Nächste Seite (Bild ab)", "Next page (Page Down)"),
     ("Oktaeder (3D-Kantenmodell)", "Octahedron (3D wireframe)"),