Add styling import / export / reset UI

[jkemmererupgrade]

Description

Slice 2 of the 3-slice split of aws#1589 per @kmcginnes's review on that PR. This is the Styling Import / Export / Reset UI. Stacks on slice 1 (aws#1777) because it relies on the lucide:<name> storage convention slice 1 introduces.

Adds three buttons to Settings → General:

• Export Styling — downloads the current node and edge styling as a pretty-printed JSON file (graph-explorer-styling.json, 2-space indent). Icons are written as symbolic references ("iconUrl": "lucide:plane") so the file is human-readable and version-controllable.
• Import Styling — accepts a styling JSON, validates it via Zod, applies it as the active styling, and stores it as the reference baseline. The schema accepts a shorthand "icon": "<name>" field that gets converted to "iconUrl": "lucide:<name>" on import.
• Reset All Styling — resets every node and edge style. If a styling file has been imported this session, restores to those values; otherwise reverts to hardcoded defaults.

The per-type Reset to Default button on NodeStyleDialog now also restores to the imported baseline (when one exists), via a new defaultStylingAtom reference copy and a shared mergeDefaultsIntoUserStyling helper.

The shared toJsonFileData utility gets an optional indent parameter (default: compact, to preserve existing callers' behavior). Slice 2's exporter passes indent: 2.

File format notes (slice 2 ↔ aws#1660)

Slice 2's exported format is intentionally a subset of what #1660's styles: section will look like — same { vertices, edges } record-of-types shape, same lucide:<name> icon refs. When the unified config lands, a slice-2 export can drop straight into #1660 under styles:.

Deliberately NOT in this PR

• No /defaultStyling static route in app.ts — that's slice 3, folded into the unified config (Unified server configuration file aws/graph-explorer#1660) per kmcginnes's direction.
• No auto-fetch in AppStatusLoader.tsx — slice 3.
• No example/defaultStyling.json or Docker-mount docs — slice 3.

Validation

• pnpm run check:types — clean
• pnpm run check:lint — 0 errors / 0 warnings (oxlint)
• pnpm run check:format — clean (oxfmt)
• pnpm test — 1787 / 1787 pass
• pnpm coverage — all four thresholds met (statements 65.19%, branches 45.47%, functions 58.81%, lines 72.59%); branches floor auto-bumped from 45 → 45 (genuine improvement from new tests).
• Manually validated against a live Neptune backend (k8s-port-forwarded). Confirmed: vertex labels/icons/colors apply on import; edge labels/colors apply on import (after correcting the test file's edge names to match the deployed schema); per-type Reset restores to the imported baseline; full Reset works both with and without an import; round-trip Export → Import preserves all styling including lucide:<name> icon refs.

Related Issues

• Slice 2 of Add defaultStyling.json support for persistent per-type vertex and edge styling aws/graph-explorer#1589
• Stacks on Add built-in Lucide icon library for node styling aws/graph-explorer#1777 (Slice 1)
• File format will be a subset of Unified server configuration file aws/graph-explorer#1660 (Slice 3 / unified config)

Check List

• I confirm that my contribution is made under the terms of the Apache 2.0 license.
• I have verified pnpm checks passes with no errors.
• I have verified pnpm test passes with no failures.
• I have covered new added functionality with unit tests if necessary.
• I have updated documentation if necessary (docs/features/settings.md, docs/features/graph-view.md).

[kmcginnes]

@jkemmererupgrade It looks like I'm not allowed to change the target of this PR now that the original target is merged. Could you update it for me?