client: skip vault sign-out broadcast when nothing existed

[dolonet]

Summary

confirmSignOut called ensureVaultId() (the minting variant) at the top. On a fresh tab where the user clicks Sign Out without ever having signed in, that minted a fresh vault_id just to delete it on the next line — and worse, _broadcastSignedOut still fired unconditionally, telling sibling tabs (which may have a live unrelated vault session) to invalidate caches, tear down their panes, and set _vaultRecentlySignedOut (blocking any subsequent saves until they hit doConnect).

User-visible impact: open one tab with an active vault session, open a second clean tab, click Sign Out on the second tab → the first tab's vault panes drop with "Vault key missing" and the next save attempt silently bails.

Fix

Switch to the non-minting ensureVaultIdIfPresent(). Record preexisting (truthy iff there was actually a vault to sign out of) and gate the four broadcast-shaped side-effects on it:

• _disconnectAllVaultPanesForNoKey()
• invalidateVaultCache()
• _vaultRecentlySignedOut = true
• _broadcastSignedOut()

Local-only cleanup (_idbDelete of K + vault_id, saveSaved([]), sessionStorage.removeItem, pane manifest filter) still runs unconditionally as defensive housekeeping — the user did click Sign Out, the local list might have orphan rows from an older session, no reason to skip the scrub.

Toast copy softens to "No saved credentials in this browser to remove" on the empty path so the click isn't silently no-op.

Tests

Two regression tests in the same Vault: Sign out of this browser block:

• sign out: empty-vault path does NOT mint vault_id or broadcast — fresh tab, no ensureVaultId calls, click Sign Out, assert broadcasts === 0 (BroadcastChannel.postMessage capture), IDB vault_id still empty, _vaultRecentlySignedOut === false.
• sign out: populated-vault path DOES broadcast to siblings — counter-test for the gate: populate vault, click Sign Out, assert broadcasts === 1 and the flag IS set.

Test plan

• cd tests/frontend && node test_connect.js — 497 / 0
• python3 test_server.py -q — 420 / 0 (unchanged; client-only fix)
• Manual: open tab A with active vault session, open tab B clean, Sign Out on tab B, verify tab A's session stays alive

Refs #67 (follow-up from holistic review).