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.

ui/e2e/game.spec.ts

@@ -16,6 +16,25 @@ async function openGame(page: Page): Promise<void> {
   await expect(page.locator('.pane')).toHaveCount(1);
 }
 
+test('offline shows the Connecting indicator and softly disables server actions (Stage 17)', async ({ page }) => {
+  await openGame(page);
+  // The exchange/draw tab is a server action; on my turn with tiles in the bag it is live.
+  const draw = page.locator('.tab').first();
+  await expect(draw).toBeEnabled();
+  await expect(page.getByText('Connecting…')).toHaveCount(0);
+
+  // Drop the connection (mock-only hook): the header swaps the title for the spinner +
+  // "Connecting…", and the server action goes inert.
+  await page.evaluate(() => (window as unknown as { __conn: { offline(): void } }).__conn.offline());
+  await expect(page.getByText('Connecting…')).toBeVisible();
+  await expect(draw).toBeDisabled();
+
+  // Reconnect: the indicator clears and the action is live again.
+  await page.evaluate(() => (window as unknown as { __conn: { online(): void } }).__conn.online());
+  await expect(page.getByText('Connecting…')).toHaveCount(0);
+  await expect(draw).toBeEnabled();
+});
+
 test('placing a tile and confirming via ✅ commits the move', async ({ page }) => {
   await openGame(page);
   await page.locator('.rack .tile').first().click();