fix(connection): fix 6 correctness bugs in auth/connect/diagnostics flow (#131)

1. buildRequestHeaders: UTF-8-encode Basic-auth credentials before btoa()
   so non-ASCII usernames/passwords don't throw (Hermes' btoa is Latin1-only
   and the throw was an unhandled rejection that hung the connect spinner).

2. diagnostics classify(): check root.ok (server reachable) before
   !internet.ok, so a reachable-but-failing server (e.g. wrong auth) is no
   longer misdiagnosed as "no internet" just because the public-internet
   probe also failed (captive portal, Tailscale-only network, etc).

3. sdk.ts createClient: strip trailing slashes from baseUrl once, so a
   trailing-slash URL from Advanced mode / Edit screen doesn't produce a
   double slash on every request path.

4. add.tsx / [id].tsx: wrap addConnection/updateConnection in try/catch so
   a SecureStore failure after a successful test resets the spinner and
   shows an alert instead of hanging forever. Adds
   connection.shared.alerts.saveFailedTitle/saveFailedMessage (en + zh-Hans).

5. add.tsx / [id].tsx: build the diagnostics probe's auth with buildAuth()
   instead of a hand-rolled expression, so the probe reproduces the real
   request's credentials (previously Quick Connect's password-only case
   sent no auth to the probe at all).

6. add.tsx handleQuickConnect: stop sending the shared `username` state,
   which could carry a stray value typed earlier in Advanced mode and
   silently override the "opencode" default after "Back to Quick".


Claude-Session: https://claude.ai/code/session_01T12AhSnQVrSxNnvwfCx2z6

Co-authored-by: engineer <engineer@macbookpro.lan>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

src/lib/i18n/zh-Hans.json

@@ -146,7 +146,9 @@
         "invalidUrlTitle": "地址无效",
         "invalidUrlMessage": "请输入包含 http:// 或 https:// 的完整地址，例如 http://192.168.1.100:4096",
         "connectionFailedTitle": "连接失败",
-        "unknownError": "未知错误"
+        "unknownError": "未知错误",
+        "saveFailedTitle": "无法保存连接",
+        "saveFailedMessage": "连接测试成功，但保存失败，请重试。"
       }
     },
     "add": {