feat(ui): room list loading/migrating state (#397)

[sanity]

Problem

During startup load — and especially during a delegate / room-contract migration — the River room list rendered blank with no loading or "migrating" indicator, which reads to users as "all my rooms are gone." Reported by Ivvor after the stdlib-0.8 River republish (delegate + room-contract re-key).

The ROOMS map populates one room at a time as each per-room slot GET returns, and on a re-keyed (empty) current delegate the rooms only arrive later via legacy migration. So an empty map does not mean "no rooms" — it usually means "still loading." Today the empty map fell straight through to a blank list (or, worse, could read as the empty state), with no way to tell "loading" from "you have no rooms."

Closes #397.

Approach

Give the room list a tri-state (ui/src/components/room_list.rs):

• loading / migrating → a quiet inline spinner + Loading your rooms… (initial load) or Restoring your rooms… (migration, driven by the existing is_legacy_migration_in_progress() localStorage flag);
• load resolved AND genuinely empty → the existing No rooms yet create/join call-to-action;
• populated → the list (unchanged).

The genuinely-empty CTA now only shows once the initial load has actually resolved, tracked by a new INITIAL_ROOMS_LOADED signal (ui/src/components/app.rs) set at the terminal points of load_rooms_per_room (response_handler.rs). Because the legacy-migration probe is fire-and-forget (a fresh user with no legacy delegate gets no response at all), the empty-delegate branch waits a bounded settle (LEGACY_PROBE_SETTLE_MS = 3s, legacy delegates are node-local) before declaring "loaded & empty" — so a migrating user never flashes the empty state during the probe gap, while a fresh user gets the CTA after a short, one-time wait.

The signal only ever transitions false → true, so a WebSocket reconnect never flashes the list back to "loading."

The tri-state decision lives in a pure room_list_display(has_rooms, migrating, initial_loaded) fn (unit-tested), consumed inline in RoomList().

UI-only — no contract/delegate WASM change, no re-key. No republish in this PR; publishing is a deliberate follow-up after the look is approved.

Aesthetics — please review the look

Restrained, near-monochrome, theme-aware; the spinner reuses the existing per-room border-text-muted style (no accent, no banner). Screenshots (light + dark) of the migrating and empty states:

• Migrating (light): quiet spinner + "Restoring your rooms…"
• Empty (light): muted comments icon + "No rooms yet" / "Create a room or join one with an invite code."

(Screenshots attached in the PR thread / provided to the reviewer. The red WebSocket pill visible at the bottom of the dev screenshots is the pre-existing ConnectionStatusIndicator — this is a no-sync dev build with no node; it is unrelated to this change.)

Testing

• New pure-fn unit tests (room_list_display) pin the crux: an empty map during migration or before load resolves is never the empty CTA; only a resolved-and-empty map is (cargo test -p river-ui --bins, 393 passed).
• cargo clippy clean (wasm, no-sync and example-data,no-sync); cargo fmt clean.
• Verified visually via a dev-only ?room_list_state=migrating|loading|empty|populated URL override (compiled out of production — example-data/wasm-gated), screenshotted in light + dark.

[AI-assisted - Claude]

🤖 Generated with Claude Code

https://claude.ai/code/session_01CD8VBzb8qsDvdcPdF6TNQG