Allow selecting patch line (ranges) and linking to them

[jpenilla]

closes #35

[github-actions]

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Name	Status	Preview	Last Commit
diffs	✅ Ready (View Log)	Visit Preview	57420e5

[Copilot]

Pull request overview

This PR implements functionality to allow selecting patch line ranges and linking to them via URL hashes. The main changes include migrating from Svelte actions to attachments, adding selection state management, and implementing URL-based navigation to specific file lines.

• Migrates event handlers from svelte/action to svelte/attachments API
• Implements line selection functionality with URL hash support for linking
• Adds visual indicators for selected lines in the diff viewer

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
Sidebar.svelte	Migrates focusFileDoubleClick from Action to Attachment; adds selection clearing on file double-click
FileHeader.svelte	Adds selectHeader function to set file selection when clicking file headers
DiffWrapper.svelte	Binds selection state between viewer and ConciseDiffView component
util.ts	Converts resizeObserver from Action to Attachment pattern
open-diff-dialog.svelte.ts	Adds linkable flag to diff metadata; preserves URL hash in navigation
diff-viewer.svelte.ts	Implements selection state management, URL hash parsing/writing, and jump-to-selection logic
ShikiThemeSelector.svelte	Updates resizeObserver usage to new Attachment API
concise-diff-view.svelte.ts	Adds LineRef types, selection logic, line reference parsing/writing functions
ConciseDiffView.svelte	Implements selection UI with visual indicators and jump-to-selection behavior

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