feat(webui): archive session support, close dialog, taller header

[edwin-zvs]

Summary

• Archived session grouping: Session list now hides archived sessions behind collapsible ▸ N archived disclosure rows, matching TUI behaviour. Each ungrouped section and each project group gets its own row; clicking toggles expand/collapse. Revealed archived sessions render dimmed (opacity 0.55).

• Close-session dialog: The × toolbar button now opens a slide-up sheet with Archive and Delete options instead of a bare window.confirm(), matching the TUI's [a]rchive / [d]elete / [N]cancel prompt. Archive fires session.archive (terminate + hide, keep history); Delete fires session.delete (permanent).

• Taller header: Header height increased from 36 px to 44 px.

Test plan

• Archive a session from the TUI; confirm it disappears from the webui session list and a ▸ 1 archived row appears in the correct section (ungrouped or under its project)
• Click the disclosure row; confirm it expands to show the archived session dimmed, and collapses again on second click
• Archive a session that belongs to a project group; confirm its disclosure row appears indented under the group header
• Click × on an active session; confirm the slide-up sheet appears with Cancel / Delete / Archive buttons
• Choose Archive in the sheet; confirm the session is archived and moves to the disclosure row
• Choose Delete in the sheet; confirm the session is removed entirely
• Choose Cancel; confirm nothing happens
• Verify header is visibly taller than before