Make header logo alt text configurable via branding config

[dougchestnut]

Summary

Lets each deployment set the header logo’s accessible name in branding config, instead of hard-coding alt="Logo".

• Adds optional branding.logoAlt next to branding.logo.
• Header <img id="brand-logo"> uses that value.
• If logoAlt is missing, alt stays "Logo" so existing config.json files keep working.
• Dev, Mirage, and sample configs set logoAlt to "Eclipse Foundation" for the bundled Eclipse mark.

Fixes eclipse-pass/main#1260

How to test

1. Run pass-ui locally (Mirage / Vite config.json is enough).
2. Inspect #brand-logo — alt should be Eclipse Foundation.
3. Remove logoAlt from the served config, reload — alt should be Logo.
4. Set logoAlt to another string, reload — the attribute should match.
5. Confirm the logo still links to branding.homepage.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[markpatton]

This makes sense to me and it worked locally. I will ask someone with a better sense of pass-ui to do a review.

[jaredgalanis]

Looks good!