Allow annotation creation without a prior selection

[david-yz-liu]

Proposed Changes

Enables graders to create annotations in MarkUs even when no region has been explicitly selected in the file viewer. Previously, all annotation creation options in the right-click context menu were disabled unless the grader had first highlighted text, dragged a box on an image, or drawn a region on a PDF.

When a grader right-clicks without a selection, the context menu now enables all annotation creation items. If the grader proceeds to create an annotation, a fallback selection is synthesized automatically:

• Text/code files — a 1-character annotation at the first non-empty, non-whitespace line (column 0–1)
• HTML/Jupyter/RMarkdown files — a 1-character annotation covering the first character of the first text node in the rendered iframe body
• Image files — a 40×40 px square centred at the point where the user right-clicked (in image pixel coordinates, clamped to image bounds)
• PDF files — a 40×40 px square centred at the point where the user right-clicked on the page (in percentage coordinates scaled by 100,000, with inverse-rotation correction applied)

If no fallback can be synthesized (empty file, HTML with no text content, PDF with no loaded pages, image with no dimensions), an alert is shown: "Cannot create annotation: the file has no content to annotate."

This change also fixes a pre-existing crash: window.annotation_manager is null for HTML files, causing the original beforeOpen handler to throw TypeError every time the context menu was opened on an HTML file.

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Associated documentation repository pull request (if applicable)

Type of Change

Type	Applies?
🚨 Breaking change (fix or feature that would cause existing functionality to change)
✨ New feature (non-breaking change that adds functionality)	X
🐛 Bug fix (non-breaking change that fixes an issue)	X
🎨 User interface change (change to user interface; provide screenshots)
♻️ Refactoring (internal change to codebase, without changing functionality)
🚦 Test update (change that only adds or modifies tests)
📦 Dependency update (change that updates a dependency)
🔧 Internal (change that only affects developers or continuous integration)

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