fix: improve config loading resilience for non-localhost access (#510) (#658)

* fix: improve config loading resilience for non-localhost access (#510)

- Add DEFAULT_CONFIG fallback to always return valid config even if fetch fails
- Implement retry logic with exponential backoff (max 2 retries) to handle transient failures
- Add 5-second fetch timeout to prevent hanging on unreachable backends
- Improve error logging with clear messages about config fetch status
- Always return DeerFlowConfig (never null) to prevent UI rendering issues
- Add safety checks in input-box component to verify reasoning models before access
- Improve type safety: verify array length before accessing array indices
- Add comprehensive documentation in .env.example with examples for different deployment scenarios
- Document NEXT_PUBLIC_API_URL variable behavior and fallback mechanism

* Apply suggestions from code review

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* fix: add nullish coalescing to prevent TypeScript error in input-box

- Add ?? operator to handle potential undefined value when accessing reasoning[0]
- Fixes TS2322 error: Type 'string | undefined' is not assignable to type 'string | number | Date'

---------

Co-authored-by: Willem Jiang <143703838+willem-bd@users.noreply.github.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

web/src/app/chat/components/input-box.tsx

@@ -214,7 +214,7 @@ export function InputBox({
       </div>
       <div className="flex items-center px-4 py-2">
         <div className="flex grow gap-2">
-          {config?.models.reasoning?.[0] && (
+          {config?.models?.reasoning && config.models.reasoning.length > 0 && (
             <Tooltip
               className="max-w-60"
               title={
@@ -226,7 +226,7 @@ export function InputBox({
                   </h3>
                   <p>
                     {t("deepThinkingTooltip.description", {
-                      model: config.models.reasoning?.[0] ?? "",
+                      model: config.models.reasoning[0] ?? "",
                     })}
                   </p>
                 </div>