Add diff hunk navigation and copy

[kcosr]

Adds hunk-level navigation and copying in diff view:

• { / } move between selected diff hunks across the stacked all-files diff
• the active hunk is marked with a thin gutter line
• y copies the selected hunk diff
• Y copies the selected file diff
• commands are also available from the command palette

Validation:

• bun run typecheck
• bun run lint
• bun run format:check
• bun test