feat: add three-mode diff view

[sam-phinizy]

Summary

• Adds diff viewing with three modes: Split (side-by-side), Unified (inline additions/deletions with dual gutters), and Highlights (additions-only overlay on existing editor)
• Rust backend computes diffs via similar crate with Patience (default) and Myers algorithms, resolves git refs via git2
• Always-visible diff mode toggle in the editor toolbar — click to enter/exit diff mode
• Ref picker with branch/tag/commit dropdowns and swap button
• Diff commands in command palette (Cmd+K)
• Auto-collapse file tree in split mode, re-diff on file switch
• Annotation blocking on delete lines in unified mode
• "No changes" message when files are identical between refs

New files

• src-tauri/src/commands/diff.rs — compute_diff and list_refs Tauri commands
• src/lib/stores/diff.svelte.ts — Diff state store
• src/lib/codemirror/diff.ts — CodeMirror decorations, ghost lines, dual gutters, scroll sync
• src/components/DiffEditor.svelte — Prop-driven CodeMirror instance for diff views
• src/components/DiffModeToggle.svelte — Segmented control for split/unified/highlights
• src/components/DiffRefPicker.svelte — Base/target ref dropdowns with swap

Test plan

• Open a file with uncommitted changes, click Split/Unified/Highlights in toolbar
• Verify split mode shows two side-by-side editors with scroll sync
• Verify unified mode shows inline diff with dual line number gutters
• Verify highlights mode overlays green on added lines in the existing editor
• Switch refs via ref picker, verify diff recomputes
• Click swap button, verify refs swap and diff updates
• Switch files while in diff mode, verify diff recomputes
• Click active mode to exit diff, verify normal editor restores
• Open identical file, verify "No changes" message appears

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