Update navigation and add sponsor date tracking

[Masstronaut]

Summary

• Updated header navigation with new links and improved mobile experience
• Added historical date tracking for all sponsors to enable future filtering

Changes

Navigation Updates

• Added "Speak at SeattleJS" link pointing to /speak page
• Replaced outdated "SeattleJS Conf 2023" link with "CascadiaJS" linking to cascadiajs.com
• Increased mobile breakpoint from 768px to 1080px for better hamburger menu experience

Sponsor Data Enhancement

• Added dateAdded field to all sponsors with accurate historical dates
• Updated TypeScript interface to include required dateAdded field
• Prepared foundation for future sponsor filtering by recency

Test plan

• Verify navigation links work correctly on desktop and mobile
• Confirm hamburger menu appears on screens 1080px and below
• Check that TypeScript compilation passes
• Validate sponsor data structure is preserved

[andrewiggins]

For sponsor filtering we probably also want to consider when the last event they sponsored was. I believe we track that?

[Masstronaut]

For sponsor filtering we probably also want to consider when the last event they sponsored was. I believe we track that?

Oh looks like we track them in events.json, but haven't updated the file since November 2024. I was using the date they were added to the sponsor list as a proxy for when they last sponsored, but I think better hygiene on the event.json and using that is probably better