[fix]: support Notion uploaded video/audio playback and fix heading background color width

[amaran-th]

Description

1. Notion uploaded video/audio playback (closes #405)

notion-client's addSignedUrls skips attachment:uuid:filename format URLs, which Notion uses for directly uploaded files. As a result, the browser receives an unsigned URL it cannot load.

• Added /api/notion-file proxy route that calls getSignedFileUrls on each request to get a fresh signed URL
• In getRecordMap, for video/audio blocks with attachment: source URLs that were skipped by notion-client, the proxy URL is injected into signed_urls so react-notion-x can render them correctly

Before	After

2. Heading block background color spans full width (closes #412)

react-notion-x applies display: inline-block to .notion-h, and the parent container uses align-items: flex-start, causing heading elements to only stretch to text width. Block background colors appeared clipped to the text area.

• Overrode with display: block; width: 100% on .notion-h

Before	After

3. ProfileCard text center alignment (closes #161)

Multi-line text in the ProfileCard was not centering properly on desktop.

• Added text-align: center to the card's flex column container

Before	After

Related tickets

#161
#405
#412

PR Checklist

• I have read the Contributing Guide
• I have written documents and tests, if needed.

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