Proposal: Add colons to h2 headings in about:preferences & about:addons

### Is your feature request related to a problem?

I think, that it would be better to have colons punctuation in the end of `

` headings in about:preferences & about:addons

#### List of h2 headings in `about:preferences`:

General tab:

- Startup
- Import Browser Data
- Tabs
- Web site appearance
- Colors
- Fonts
- Zoom
- Language
- Downloads
- Applications
- Digital Rights Management (DRM) Content

Home tab:

- New Windows and Tabs
- Firefox Home Content

Search tab:

- Search Bar
- Default Search Engine
- Search Suggestions
- Search Shortcuts

Privacy & Security tab:

- Enhanced Tracking Protection
- Cookies and Site Data
- Logins and Passwords
- History
- Address Bar
- Deceptive Content and Dangerous Software Protection
- Certificates
- HTTPS-Only Mode

Sync tab:

- Take Your Web With You
- Device Name

Headings in dialogues:

- Text and Background
- Link Colors
- Fonts for
- Configure Proxy Access to the Internet

#### List of h2 headings in `about:addons`:

All headings:

- Enabled
- Disabled
- Saved themes
- Recent Updates

### The solution I'd like

Add colons punctuation to the end of listed headings to `about:preferences` & `about:addons`.

With exception to title headings in dialogues. And with exception to “Enable secure DNS using:” (because it already has colon)

How it should look like:
- Startup:
- Import Browser Data:
- Tabs:
- Web site appearance:
- (etc…)

### Screenshots or video

_No response_

### Alternatives I've considered

_No response_

### Additional context

_No response_

Start by locating the about:preferences and about:addons heading definitions in the repository's SCSS and UI resources. Review the listed h2 headings and verify the dialogue-title and existing “Enable secure DNS using:” exceptions. Done means the requested headings consistently end with colons without changing those exceptions.

Written by the indexing model from the issue text.