fix: prevent non-SSE stream pipe crash and cross-IdP account overwrites (#2244)

- streamingHandler: when upstream returns non-SSE/JSON (e.g. Cloudflare
  5xx HTML), read body, sanitize <title>, notify streamController and
  return a clean JSON error instead of crashing the pipe.
- connectionsRepo: dedup OAuth connections on (email + username) so
  cross-IdP accounts sharing an email no longer overwrite each other;
  workspace providers keep workspace-id matching.
- kimchi: bump User-Agent to 0.1.50, add svg asset + browser-login
  service, and 21 unit tests.

Co-authored-by: Cursor <cursoragent@cursor.com>

public/providers/kimchi.svg

@@ -0,0 +1,4 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="104" height="104" viewBox="0 0 104 104">
+<circle cx="52" cy="52" r="52" fill="#FF521D"/>
+<path d="M68.4459 24.9438C69.8639 24.1762 71.6471 23.7178 73.4821 24.2163C73.8278 24.2966 74.1339 24.3916 74.4362 24.5024L75.9186 25.0464L75.5104 26.5727C75.3469 27.1827 75.202 27.723 75.0387 28.3325L74.6442 29.8042L73.1354 29.5972C73.0242 29.5819 72.9246 29.5753 72.8082 29.5747C72.2474 29.5865 71.7542 29.8645 71.2662 30.6772C70.8883 31.307 70.5939 32.1486 70.3815 33.1079C74.871 36.0331 77.8444 41.0994 77.8444 46.8638C77.8443 49.5292 77.2072 52.0505 76.0758 54.2798C71.5108 64.3996 54.1783 84.88Line truncated
+</svg>