feat(swa): pin title and info button in sticky header bar

[Ryback2501]

Summary

• Remove the "A minimal Nostr client" subtitle and pin the Lobstr title and info button in a sticky header bar that stays at the top while the page content scrolls beneath it.

Changes

SPA (swa/)

• index.html — drop the subtitle paragraph; wrap the title and info button in a .header-bar row inside <header>.
• style.css — make <header> position: sticky; top: 0 with a surface background and bottom border (z-index: 50, below the z-index: 100 modals); move the page padding from body onto main so the bar spans full width; centre the .header-bar row at 640px; remove the now-unused .subtitle rule and the button alignment hack; reduce the title to 1.5rem to fit the compact bar.

Test plan

• Load the app: the title and info button appear in a bar pinned to the top of the page.
• Scroll the feed: the header stays fixed while content scrolls beneath it.
• Confirm the subtitle no longer appears under the title.
• Open the info modal via the button: it renders above the header (not obscured).
• Verify the title aligns horizontally with the panels below it.

Validation

• /validate-full: lint PASS, unit PASS (290/290), integration/e2e PASS (placeholders), docker-smoke PASS (build + run + / → 200).
• Self-review verdict: GO.