feat: endless canvas grows only to the right and down (Issue #30)

The view stops 10 mm left of and above the start frame, or above and left
of content that lies further out in notebooks from 0.29/0.30, so a new
endless notebook starts at the top left like a sheet of paper.

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

README.md

@@ -52,7 +52,7 @@ The interface is available in English and German; ink-notes follows the system l
 ### Pages and view
 - **Backgrounds:** blank, squared (5 mm), ruled (7.5 mm), dot grid (5 mm). The **color of squares, lines and dots** is selectable (light, medium, strong, blue, green) and also applies to exports.
 - **Paper color** on screen: white, cream, dark gray, black; on dark paper, dark pen colors can optionally be brightened. Saving and exporting always use white paper.
-- **Continuous scrolling, endless canvas or fixed pages, per notebook:** chosen when creating a notebook (default: continuous; or endless) and switchable in the ☰ menu – where fixed pages are available too (page by page with arrows in the toolbar). In continuous mode the pages hang seamlessly below each other with a dark line marking the boundary. A quick swipe keeps gliding, and the scrollbar on the edge opposite the toolbar jumps through long PDFs with the page number shown. Writing in the bottom fifth of the last page adds a new page automatically. Endless is a single borderless canvas you can write on in every direction; the PDF export only contains the area you used.
+- **Continuous scrolling, endless canvas or fixed pages, per notebook:** chosen when creating a notebook (default: continuous; or endless) and switchable in the ☰ menu – where fixed pages are available too (page by page with arrows in the toolbar). In continuous mode the pages hang seamlessly below each other with a dark line marking the boundary. A quick swipe keeps gliding, and the scrollbar on the edge opposite the toolbar jumps through long PDFs with the page number shown. Writing in the bottom fifth of the last page adds a new page automatically. Endless is a single borderless canvas that grows to the right and down as far as you like, starting at the top left; the PDF export only contains the area you used.
 - **Insert and delete pages:** ➕ in the toolbar inserts an empty page after the current one. Delete it in the ☰ menu (with confirmation); the most recently deleted page can be restored there while the app is running.
 - **Zoom** from 25 % to 1000 % (`Ctrl + mouse wheel`, two fingers, `Ctrl + +/-/0`), pan with the middle mouse button, `Space + drag` or one finger.
 - **Pages** can be appended or inserted right after the current page, and given **page titles**; the page overview shows thumbnails or a table of contents. Page titles become bookmarks in the PDF.