fix(chat): repair the line-numbered chat template merged by #336

[Ayinkx]

Problem

app/templates/chat/index.html on main is not a template any more - it is a patch pasted into a file. The squash merge of #336 (79a2442, "fix: streaming indicator and typing feedback for chat") carried line numbers into the file:

-<%extends "base.html" %}          <- opening tag corrupted
-2<aside class="chat-sidebar">     <- its own line number, no indentation
-3    <div class="sidebar-header">
-5      <button ...>+ New</button> <- also gained a U+200B zero-width space

121 of 135 lines are prefixed with their source line number and two zero-width spaces were introduced. The template still compiles and every test still passed, so the corruption has been live on main since 2026-10-06 - the digits simply render into the chat page.

I hit this while resolving conflicts on the open chat PRs (#346, #318, #319, #342, #347, #348, #354, #355): every one of them conflicts with this file, and resolving "against main" would mean resolving against the corrupted text.

Fix

Restore the last known-good revision (f0dbbde, the commit before #336) and re-apply the only semantic change #336 made to this template: the per-conversation

<span class="conversation-status" hidden aria-hidden="true"></span>

that app/static/js/chat.js writes the streaming indicator into. Everything else in that commit (app/chat/api.py, app/chat/routes.py, app/services/streaming.py, app/static/js/chat.js, tests/test_streaming.py) is unaffected and untouched.

Verified by diffing the restored file against main with the numbers stripped - the only remaining differences are whitespace, the corrupted <%extends, and the zero-width spaces.

Regression guard

New tests/test_template_hygiene.py:

• fails if any file under app/templates/ contains a line starting with a 1-4 digit number followed by markup (the exact shape this corruption took). Only chat/index.html is flagged by it on main, so it does not create false positives.
• asserts the chat page still starts with {% extends "base.html" %} and keeps the conversation-status span.

pytest tests/test_template_hygiene.py tests/test_chat.py -q
..................................  66 passed

ruff check .   -> All checks passed!
black --check . -> all files unchanged