加入import功能，给sidepanel加了一点背景UI。

[martinhamburger]

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[Copilot]

Pull request overview

This PR adds a JSON import pathway to complement existing export functionality, and refreshes sidepanel Settings UI with more “card/surface” styling (borders, shadows, and grouped Support rows).

Changes:

• Add “Import JSON” flow in the Data Management panel (file picker → confirm → import → refresh overview).
• Extend messaging/types/service plumbing to support a new IMPORT_DATA request that runs importAllData and triggers vector re-indexing.
• UI polish: Settings support section gets a grouped shell + reveal panel; various surfaces gain consistent background/border/shadow styling.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
frontend/src/style.css	Adds/adjusts surface styles (borders/backgrounds/shadows) and introduces settings support shell/reveal styles.
frontend/src/sidepanel/pages/SettingsPage.tsx	Refactors/rewraps Settings UI markup and applies new support-shell layout/classes.
frontend/src/sidepanel/components/DataManagementPanel.tsx	Adds “Import JSON” control and import handling logic (confirm, call importData, feedback).
frontend/src/offscreen/index.ts	Adds IMPORT_DATA request handling (calls importAllData, triggers vectorization).
frontend/src/lib/types/index.ts	Adds ImportDataResult type and updates formatting.
frontend/src/lib/services/storageService.ts	Adds importData(content) service that sends IMPORT_DATA to offscreen and emits VESTI_DATA_UPDATED.
frontend/src/lib/messaging/protocol.ts	Adds IMPORT_DATA to request/response typing and maps it to ImportDataResult.
frontend/src/background/index.ts	Adds IMPORT_DATA handling in the offscreen-target handler and kicks off vectorization after import.

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