Stage 17 #2: Connecting indicator + auto-retry, instead of red toasts

Connectivity failures become state, not a toast on every attempt. A global online
signal (lib/connection.svelte.ts) flips on a transport unavailable / rate_limited and
on the live stream's drop, driving a pure-CSS header spinner + 'Connecting…' in place
of the title and softly disabling the in-game server actions (commit / exchange / pass
/ hint; local board/rack/reset stay live).

- transport: exec auto-retries with capped exponential backoff — every op on a
  rate-limit (rejected before processing, safe), reads only on unavailable (a mutation
  is never blindly re-sent, to avoid double-applying one whose response was lost; its
  button is disabled while offline so the player re-issues on reconnect). A reachability
  watcher (profile.get probe) and any successful traffic clear the signal.
- the old red error.unavailable toast is gone (handleError suppresses connection codes;
  the indicator replaces it). A server-data screen still opens with the spinner and
  fills on reconnect (global indicator + read auto-retry), so navigation is never dead.
- pure retry policy unit-tested (retry.ts); a mock-only window.__conn hook drives a
  Chromium+WebKit e2e (indicator shows offline, the action disables, both clear on
  reconnect). Full suite + build green.
- docs: ARCHITECTURE transport note, FUNCTIONAL (+ _ru), PLAN tracker (incl. #1 — the
  bot already drains all updates, no change).

Also records #1 as investigated/no-change in PLAN. Other server-action buttons (chat
send, profile save, …) still degrade to a safe no-op offline; visual disable is easy to
extend.

docs/ARCHITECTURE.md

@@ -89,7 +89,15 @@ dropped). Horizontal scaling is explicit future work.
   auth operations are unauthenticated and return the minted token. A unary
   operation's domain outcome rides back in `ExecuteResponse.result_code` (HTTP
   200); only edge failures (rate limit, missing session, unknown type, internal)
-  surface as Connect error codes.
+  surface as Connect error codes. The client (Stage 17) treats a connectivity edge failure as
+  **state, not a per-call toast**: a transport `unavailable` or a `rate_limited` flips a global
+  `online` signal that drives a header **"Connecting…"** spinner and softly disables proactive
+  actions, and the transport **auto-retries with capped exponential backoff** — every op on a
+  rate-limit (the gateway rejected it before processing, so it is safe), but only **read-only**
+  ops on `unavailable` (a mutation is never blindly re-sent, to avoid double-applying one whose
+  response was lost — its button is disabled while offline and the player re-issues it on
+  reconnect). A reachability watcher (a lightweight `profile.get` probe) clears the signal when no
+  other traffic is in flight; the live `Subscribe` stream's drop/recovery feeds the same signal.
 - **Alphabet on the wire (Stage 13)**: live play exchanges **alphabet indices**, not
   concrete letters. The rack (`StateView.rack`), the `SubmitPlay`/`Evaluate` tiles, the
   `Exchange` tiles and the `CheckWord` word are `ubyte` indices into the variant's alphabet