Fix billing null guards

[HarshMN2345]

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[greptile-apps]

Greptile Summary

This PR adds defensive null/optional-chaining guards across four files to prevent TypeError crashes when billing plan data or user account data is not yet populated. No behavioural changes are introduced — each fix is a minimal, targeted safeguard.

• billing.ts: billingIdToPlan now accepts null | undefined and returns early; checkForUsageFees and checkForProjectLimitation propagate that via optional chaining; checkForNewDevUpgradePro gains guards on both billingPlanDetails and account.$createdAt.
• tab.svelte: handleKeyDown gains three early-returns guarding against closest() returning null and indexOf returning -1.
• containerHeader.svelte / newDevUpgradePro.svelte: Optional chaining added on $currentPlan?.usage and billingPlanDetails?.supportsCredits respectively.

Confidence Score: 5/5

All changes are additive null guards with no logic alterations — safe to merge.

Every change is a narrow defensive fix: optional chaining, nullish coalescing, or an early return. The previously flagged unguarded access in checkForProjectLimitation (billingPlan.projects) has been addressed with billingPlan?.projects. No new code paths are introduced and all fallback values are conservative.

No files require special attention.

Important Files Changed

Filename	Overview
src/lib/stores/billing.ts	Adds null/undefined guards across billingIdToPlan, checkForUsageFees, checkForProjectLimitation, and checkForNewDevUpgradePro — all previously raised runtime errors are addressed
src/lib/components/tab.svelte	Adds three defensive early-returns in handleKeyDown for null tabItem, null tabsList, and indexOf returning -1 — correct and safe
src/lib/layout/containerHeader.svelte	Guards Object.keys access on $currentPlan?.usage with nullish coalescing fallback to empty object
src/lib/components/billing/alerts/newDevUpgradePro.svelte	Adds optional chaining on billingPlanDetails?.supportsCredits in the template condition

_{Reviews (2): Last reviewed commit: "Guard project limitation plan lookup" | Re-trigger Greptile}