fix(usage): improve usage-query resilience and error surfacing

- useUsageQuery: retry once + keep-last-good — show the last successful
  result for up to 10min when a query fails transiently (network/timeout/
  HTTP 5xx), so a single blip no longer flips the card to red. Deterministic
  failures (auth, empty key, unknown provider, 4xx) surface immediately and
  clear the snapshot so a stale quota can't resurface after credentials change.
- bump native balance/coding-plan/subscription request timeouts 10s -> 15s
  for slow cross-border endpoints.
- coding_plan: return explicit errors ("API key is empty" / "Unknown coding
  plan provider") instead of a blank failure, mirroring balance.
- add unit tests for keep-last-good and transient/deterministic classification.

src-tauri/src/services/subscription.rs

@@ -328,7 +328,7 @@ async fn query_claude_quota(access_token: &str) -> SubscriptionQuota {
         .header("Authorization", format!("Bearer {access_token}"))
         .header("anthropic-beta", "oauth-2025-04-20")
         .header("Accept", "application/json")
-        .timeout(std::time::Duration::from_secs(10))
+        .timeout(std::time::Duration::from_secs(15))
         .send()
         .await;
 
@@ -668,7 +668,7 @@ pub(crate) async fn query_codex_quota(
         req = req.header("ChatGPT-Account-Id", id);
     }
 
-    let resp = match req.timeout(std::time::Duration::from_secs(10)).send().await {
+    let resp = match req.timeout(std::time::Duration::from_secs(15)).send().await {
         Ok(r) => r,
         Err(e) => {
             return SubscriptionQuota::error(
@@ -964,7 +964,7 @@ async fn refresh_gemini_token(refresh_token: &str) -> Option<String> {
             ("refresh_token", refresh_token),
             ("grant_type", "refresh_token"),
         ])
-        .timeout(std::time::Duration::from_secs(10))
+        .timeout(std::time::Duration::from_secs(15))
         .send()
         .await
         .ok()?;
@@ -1048,7 +1048,7 @@ async fn query_gemini_quota(access_token: &str) -> SubscriptionQuota {
                 "pluginType": "GEMINI"
             }
         }))
-        .timeout(std::time::Duration::from_secs(10))
+        .timeout(std::time::Duration::from_secs(15))
         .send()
         .await;
 
@@ -1109,7 +1109,7 @@ async fn query_gemini_quota(access_token: &str) -> SubscriptionQuota {
         .header("Authorization", format!("Bearer {access_token}"))
         .header("Content-Type", "application/json")
         .json(&quota_body)
-        .timeout(std::time::Duration::from_secs(10))
+        .timeout(std::time::Duration::from_secs(15))
         .send()
         .await;