appview/oauth: fix session cookie flags for dev mode
gorilla/sessions defaults to Secure=true and SameSite=None, which
prevents the session cookie from being sent over plain HTTP. This
caused immediate logout after a successful OAuth callback in the dev
environment (TANGLED_DEV=true).
The accounts and auth-return cookies already override these defaults
with Secure=!Dev and SameSite=Lax, but the main session cookie
(appview-session-v2) was missing the same treatment. Apply consistent
cookie options at the store level so all sessions inherit them.
Signed-off-by: Alessio Caiazza <code.git@caiazza.info>
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