[codex] polish session list/header metadata UX

[gaius-codius]

Summary

• polish session list rows and session header metadata with compact dot-separated chips
• add color-coded flavor labels, pending/thinking badges, inactive dimming, and session enter animation
• keep grouped session headers by machine/path; path only in group headers, not each session row
• fix touch + desktop expand/collapse behavior for grouped session headers
• improve new session page scrolling so create actions stay reachable
• add focused unit/Playwright coverage for the metadata polish

Scope

This PR is intentionally limited to session list/header glanceability polish and closely related UX fixes.

Not included:

• manual sorting
• group/session move menus
• sort preference persistence

Impact

• denser, clearer session metadata in sidebar and header
• better flavor/model/effort/permission/worktree visibility
• more reliable mobile expand/collapse behavior
• better create-session usability on shorter viewports

Validation

• bun run typecheck:web
• cd web && bun run test SessionList SessionHeader SessionActionMenu
• focused local Playwright/manual validation for:
  • session metadata rendering
  • group expand/collapse on touch + desktop
  • new-session scrolling / reachable create button

[gaius-codius]

Addressed the valid review items in 4655347 (fix: address PR review follow-ups).

Fixed:

• restored move-threshold cancel in useLongPress via onPointerMove (~8px) to avoid accidental navigation while touch-scrolling
• validated session-alive payloads with Zod, including modelMode
• quoted interpolated values in web/playwright.config.ts
• removed the enteringSessionIds bookkeeping race by computing the diff and updating the known-id set in the same memo
• removed unused index in SessionList
• merged duplicate reduced-motion rule for .animate-session-enter
• restored .happy-thread-messages > * { content-visibility: auto; contain-intrinsic-size: auto 80px; }
• added a clarifying comment in SyncEngine.applySessionConfig that modelMode is currently hub-managed metadata only and is not yet part of the CLI set-session-config RPC

Validation:

• bun run typecheck:web ✅
• cd web && bun run test SessionList SessionHeader SessionActionMenu ✅
• bun run typecheck:hub still hits pre-existing unrelated errors in hub/src/tunnel/tlsGate.ts and hub/src/web/routes/guards.ts