fix(ux): fixed-overlay header + pipe connector, profile banner/avatar, sidemenu name truncation

[dmnyc]

Summary

UX polish for the desktop header, profile page, and the user side menu. Three related fixes that ride together.

Header → fixed overlay

• Convert the desktop header from sticky-in-scroll to a fixed overlay so the bar no longer rubber-band bounces on overscroll (the page still scrolls behind it).
• Publish the header height as a --header-h CSS variable so the kitchen and community sub-tabs pin directly beneath it (no overlap, no gap).
• Add the desktop pipe connector to the left of the search bar, and move the header divider to an ::after pseudo-element (hidden at xl so the pipe takes over without a double line).

Profile page

• Banner is full-bleed below xl (square top; square bottom on mobile where there's no column edge) and a contained rounded card at xl — never wider than the top bar.
• Indent the avatar so the banner's left edge/rounded corner stays visible.
• Align the name/bio to the content column (same left margin as stats, tabs, and note body).

Side menu

• Truncate long display names instead of letting them break the side-menu header layout.

Notes

• Independent of the NIP-46 remote-signer fix (fix(nip46): remote signers (Primal) stamp events with signer pubkey, breaking uploads/zaps/auth #452); no overlapping files.
• svelte-check passes (0 errors).