feat(onboarding): auto-enable PostHog internal signal sources

[Twixes]

Summary

Make the PostHog data signal sources (Error Tracking, Support, Session Replay) opt-out during onboarding instead of opt-in. New projects land on the Signals step with these toggles already on, so the inbox starts populated without the user having to flip three switches before the next step.

How it works

• New autoEnableInternalSources method on useSignalSourceManager — for each PostHog-internal source that has no existing SignalSourceConfig, it calls the existing handleToggle(source, true) path, which creates the config server-side with enabled: true and uses the same optimistic-update path as a manual toggle.
• SignalsStep fires it once via useEffect after configs finish loading (guarded by a ref so navigating back/forward doesn't re-trigger).
• Sources that already have a config — enabled or disabled — are skipped, so an explicit user choice is never overwritten. External sources (GitHub / Linear / Zendesk) are untouched and remain opt-in (they require setup).

Test plan

• Brand-new project: open onboarding → reach the Signals step → Error Tracking, Support, and Session Replay are toggled on automatically; "Continue to setup" shows instead of "Skip for now"
• Toggle one of the three off mid-step, navigate away and back: the off state sticks (no re-enable on remount)
• Existing project that has previously disabled all internal sources: revisiting the Signals step does not re-enable them
• External sources (GitHub / Linear / Zendesk) still require setup and remain off until configured

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