Copy local file location from the preview panel

[jongio]

Summary

• add a preview shortcut to copy a repo-relative local file location
• format single lines as path:line and ranges as path:start-end
• document the new shortcut in the generated keybinding reference

Validation

• go test ./internal/panels/preview ./internal/keybindings -count=1
• go test ./... -count=1
• go vet ./...
• go build ./...
• golangci-lint run ./internal/panels/preview ./internal/keybindings --timeout 5m
• mage preflight: failed at lint because golangci-lint scanned the leftover sibling worktree .ideawt\diff-stats-footer after Windows kept temp files locked during cleanup

Closes #285