Fix connect handlers running before tab storage is created

Description

_on_handshake invokes the connect handlers via client.handle_handshake(...) and only then awaits _create_tab_storage (nicegui/nicegui.py:220-223). On a tab's first handshake there is no _tabs entry yet, so a sync connect handler runs inline before the entry exists and app.storage.tab trips its own assertion. safe_invoke catches it, so the page still renders — the rest of that handler is skipped and the only trace is a logged ERROR. (A reconnect is fine: copy_tab runs earlier and populates _tabs.)

An async handler is scheduled as a task and lands after the await, so it works. Same API, same handler body, opposite outcome depending on def vs async def; nothing in the docs for app.on_connect / client.on_connect mentions it. The documented await ui.context.client.connected() pattern is also safe, which is probably why this has gone unnoticed.

Independent of #6310 — that one is On Air only, this is the local Socket.IO path. #6332 mirrors _on_handshake into air.py, so it inherits the same ordering.

Example Code

from nicegui import app, ui

@ui.page('/')
def index():
    ui.context.client.on_connect(lambda: app.storage.tab.__setitem__('x', 1))
    ui.label('hello')

if __name__ in {'__main__', '__mp_main__'}:
    ui.run(storage_secret='x')

The page renders; the server log shows:

ERROR nicegui:app.py:187 tab storage for 3b199ba2-... should be created before accessing it
  File "nicegui/client.py", line 463, in safe_invoke
  File "nicegui/storage.py", line 177, in tab
AssertionError: tab storage for 3b199ba2-... should be created before accessing it

client.on_connect  (sync)    FAIL: tab storage for <id> should be created before accessing it
app.on_connect     (sync)    FAIL: same
client.on_connect  (async)   OK
await client.connected()     OK

All four measured via the user fixture; the sync case additionally reproduced through a real browser handshake with the screen fixture (the traceback above is from that run). Clean checkout of main 0e666fd9, and byte-identical on #6332's branch, so neither introduced it.

With redis_url set, the two OK rows dodge the assertion but are not fully safe either: _create_tab_storage installs the dict at storage.py:184 and only then awaits tab.initialize() at :187, so a resumed waiter can reach a not-yet-loaded dict.

Possible fix

Create the tab storage before the handlers run. A plain swap of the two statements is not quite enough: under Redis _create_tab_storage yields, and handle_handshake also does lifecycle bookkeeping (socket registration, cancelling the delete task, connection counting) that should not be delayed behind that await. So the split is bookkeeping first, storage next, handler invocation last. Folding the shared sequence into one Client method — the leave-or-take from #6279, raised again in #6310 — would fix both transports at once.

NiceGUI Version

3.16.0.post24.dev0+0e666fd9 (main)

Python Version

3.14.2