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>

README.md

@@ -29,6 +29,7 @@ The interface is available in English and German; ink-notes follows the system l
 - **Pen buttons** can be assigned in the ☰ menu: erase (default), select, highlighter, pen or nothing – active while the button is held. Each button appears there after it is pressed for the first time.
 - **Eraser** with three modes: whole strokes, only the part you wipe over (partial stroke), or highlighter only. The back end of the pen erases too.
 - **Virtual ruler** (`L`): move it, rotate it by its handle (snaps to 15°); strokes along an edge become straight.
+- **View only** (👁 in the toolbar): pen, finger and mouse only scroll and zoom; the tools disappear and nothing can be changed by accident. Tap 👁 again to write.
 - **Make space** (↕ in the toolbar): tap the page and drag down – everything below moves along, also on the following pages. Whatever would slide past the end of a page continues on the next one; missing pages are appended. Dragging up closes empty space. Undo reverts it in one step.
 
 ### Shape recognition