feat: add out-of-credits warning to sidebar plan badge

[prakhar1605]

What

Sidebar plan badge (Free / Starter / Pro) now shows an out-of-credits warning — a red badge state ("Out of credits" + ⚠️) with an upgrade popover ("You've run out of credits → Upgrade now"), matching the POC. It clears automatically when credits are available again.

Detection (both signals you mentioned)

• /v1/me billing payload — warning turns on when the daily or monthly bucket is exhausted (availableCredits <= 0). Same data the Settings usage bars use; no new API work.
• Usage API error — the existing matchBillingError "not enough credits" path also flips it on, covering the case where cached billing is stale.

Both feed a single warning state, so there's one source of truth.

Clearing (no polling)

• A successful cost-incurring call clears it: LLM finish-step (with usage) or a successful voice synth.
• On clear, we re-fetch /me once, and only if the warning was actually showing — so normal runs don't trigger any extra /me calls.

How it's wired

• New lib/credit-status.ts — isOutOfCredits(billing) helper + two window events (credit:exhausted / credit:replenished) with dispatch helpers (single source for the condition + event names).
• App.tsx dispatches exhausted on a not_enough_credits billing error, and replenished on a successful LLM finish-step.
• useVoiceTTS.ts dispatches replenished on a successful synth.
• sidebar-content.tsx owns the warning UI + listens to both events (billing stays authoritative; events flip it in between).
• billing-error.ts — added a kind discriminator so we only react to the credits error, not other billing errors.
• ui/popover.tsx — added PopoverArrow (for the popover pointer).

Files

• lib/credit-status.ts (new)
• App.tsx, components/sidebar-content.tsx, hooks/useVoiceTTS.ts, lib/billing-error.ts, components/ui/popover.tsx

Testing

• Forced isOutOfCredits → true: badge + popover render correctly; X dismisses popover while the indicator stays; "Upgrade now" / "Upgrade" open ?intent=upgrade.
• Dispatched credit:exhausted / credit:replenished from devtools: warning toggles; /me re-fetch fires only on clear-after-warning, and a normal run after that fires no extra /me.
• tsc --noEmit clean across renderer.

Note

The existing billing-error modal still shows alongside the new sidebar warning — per your call that they're two separate things. No change to the modal.