fix: optimistic user message echo + tabs persistence

Bug 1 (sent message didn't appear until turn end): the transcript only folded
assistant AgentEvents, so the user's own message showed only after turn-sealed
resync. Add core/chunks appendUserMessage() (provisional user chunk, superseded
on history sync) and call it in chat send() — the message now renders instantly.

Bug 2 (tabs didn't persist on refresh): the app passed { storage: undefined }
to createLocalStore, which the adapter treats as a no-op store, so nothing was
saved. Default to globalThis.localStorage. Regression test exercises the
non-injected path.

Also updated app store tests for the echo (assistant-vs-user chunk filtering).
Verified: svelte-check 0/0, vitest 288 (stable x2), biome clean, build ok.

1 files changed, 16 insertions, 0 deletions

diff --git a/src/core/chunks/reducer.ts b/src/core/chunks/reducer.ts
index 0a8ea54..d3b999d 100644
--- a/src/core/chunks/reducer.ts
+++ b/src/core/chunks/reducer.ts
@@ -166,3 +166,19 @@ export function foldEvent(state: TranscriptState, event: AgentEvent): Transcript
 		}
 	}
 }
+
+/**
+ * Optimistically append a user message to the provisional list.
+ * Flushes any in-progress accumulating chunk first (defensively).
+ * The provisional user chunk is superseded when applyHistory receives
+ * the authoritative committed chunks after a turn seals.
+ */
+export function appendUserMessage(state: TranscriptState, text: string): TranscriptState {
+	const provisional = flushAccumulating(state.provisional, state.accumulating);
+	const userChunk: Chunk = { type: "text", text };
+	return {
+		...state,
+		provisional: [...provisional, { role: "user", chunk: userChunk }],
+		accumulating: null,
+	};
+}