fix(webui): stop password managers autofilling the search box, and keep unsaved sessions startable

Two independent defects that chain into "new conversations cannot be started":
every POST /api/session/draft and /api/chat/start 404s a few seconds after
POST /api/session/new returned 200.

1. Search inputs are parsed as username fields.

index.html contains no <form> at all, but does contain password inputs (the
Settings "Access Password" panel). Chromium therefore groups the document's
unowned fields into one synthetic password form and picks the first text input
in the DOM — #sessionSearch, the sidebar conversation filter — as that form's
username field, then autofills the saved account name into it on load.
autocomplete="off" was already present; it is ignored for credential
heuristics.

The autofill fires oninput -> filterSessions() -> GET /api/sessions/search
?content=1&depth=5: a full content search the user never asked for, repeated
on every page load.

Fix: give the four search inputs type="search" plus the per-manager opt-outs
(data-1p-ignore / data-lpignore / data-bwignore / data-form-type), and give the
Settings password inputs a real <form> owner with proper autocomplete tokens so
Chromium scopes the credential form to them instead of to the whole document.
The form uses display:contents, so layout is unchanged. CSS suppresses WebKit's
native search clear button, since these fields ship their own
(#sessionSearchClear).

2. A never-persisted session is treated as evictable.

new_session() intentionally keeps a session in RAM until its first message
(#1171), so the SESSIONS cache is its only copy. _session_is_evictable()
(#4765) short-circuited on zero messages, reasoning that an empty shell "is
recreated on next access". It is not: get_session() has no recreate path and
raises KeyError, so both routes 404 and the session can never be started.

The content search from (1) pulls every hit through get_session(), which blows
past sessions_cache_max (default 300) and evicts the session being composed.
That is why this reproduces within seconds on an install with ~1700 sessions,
and why it grew worse as the session count rose.

Fix: require proof of persistence before evicting, for empty sessions too —
which is what the function's own docstring already promised ("Its full state is
already persisted to the JSON sidecar"). The zero-message branch was the single
path that bypassed it. #4765's memory bound is unaffected: sessions loaded from
disk remain evictable, and its existing tests still pass.

Trade-off: an unsaved empty session now stays resident for the process
lifetime (one per "New Conversation" click, an empty object each). That is a
deliberate application of the function's stated invariant — "slightly more RAM
for a session we are unsure about is strictly better than evicting an unsaved
session". Bounding it by age instead would also work if maintainers prefer.

Validation:
  pytest tests/test_issue4765_sessions_lru_eviction.py  ->  9 passed
  The added regression test fails on the unpatched predicate (verified by
  reverting api/models.py alone) and passes with the fix.

Note: (1) fixes what triggers the churn, (2) fixes what turns that churn into
an unstartable session. Either fix alone leaves a real defect in place, so
they are submitted together.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

api/models.py

@@ -4056,6 +4056,14 @@ def _session_is_evictable(s) -> bool:
         on-disk ``message_count`` being at least the in-memory message count.
         A metadata-only stub is inherently backed by disk, so it is evictable.
 
+    The persistence requirement holds even for a session with ZERO messages.
+    ``new_session()`` deliberately does not touch disk until the first message
+    (#1171), so between "New Conversation" and the first send, this cache is the
+    session's ONLY copy. Evicting it does not discard a recreatable empty shell:
+    ``get_session()`` has no recreate path and raises ``KeyError``, so the very
+    next ``/api/session/draft`` or ``/api/chat/start`` 404s and the session can
+    never be started. Sessions the user is still composing must stay resident.
+
     Anything we cannot positively prove is safe stays resident. Using slightly
     more RAM for a session we are unsure about is strictly better than evicting
     an active or unsaved session (task safety invariant: a half-done memory fix
@@ -4077,14 +4085,12 @@ def _session_is_evictable(s) -> bool:
     if getattr(s, '_loaded_metadata_only', False):
         return True
     in_memory_count = len(getattr(s, 'messages', None) or [])
-    if in_memory_count == 0:
-        # A zero-message session has nothing to lose. If it was never persisted
-        # (brand new, no sidecar) dropping it only discards an empty shell; the
-        # next access recreates it. If it is persisted, it is trivially clean.
-        return True
     disk_count = _persisted_message_count(sid)
     if disk_count is None:
         return False  # cannot prove it is on disk → keep it resident
+    if in_memory_count == 0:
+        # Persisted and empty → trivially clean, nothing to lose.
+        return True
     return disk_count >= in_memory_count