Revamp TorchAgent UI (#6817)

## Summary
- modern chat layout and bubbles
- bottom-pinned input stays active while streaming
- auto-scroll logic scoped to chat window
- remove WelcomeSection and update styles
- add unit tests for chat UI

## Testing
- `yarn tsc`
- `yarn test`


------
https://chatgpt.com/codex/tasks/task_e_6857d82d9764833388d3bfa6e2be5142

Author: wdvr

clickhouse_db_schema/misc.torchagent_feedback/schema.sql

@@ -1,10 +1,12 @@
-CREATE TABLE misc.torchagent_feedback (
-    `user` String,
-    `session_id` String,
-    `feedback` Int8,
-    `time_inserted` DateTime64(0, 'UTC')
-)
-ENGINE = SharedMergeTree('/clickhouse/tables/{uuid}/{shard}', '{replica}')
-PARTITION BY toYYYYMM(time_inserted)
-ORDER BY (user, session_id, time_inserted)
-SETTINGS index_granularity = 8192
+CREATE TABLE
+    misc.torchagent_feedback (
+        `user` String,
+        `session_id` String,
+        `torchagent_feedback` String,
+        `feedback` Int8,
+        `time_inserted` DateTime64 (0, 'UTC')
+    ) ENGINE = SharedMergeTree ('/clickhouse/tables/{uuid}/{shard}', '{replica}')
+PARTITION BY
+    toYYYYMM (time_inserted)
+ORDER BY
+    (user, session_id, time_inserted) SETTINGS index_granularity = 8192