Knowledge bases, and a file input that lines up

**Bases.** Documents now live in named collections rather than one flat pile,
and a chat can be pointed at particular ones — "answer from the contracts
folder" is a different question from "answer from everything I have ever
uploaded". A chat with none attached still searches everything its owner can
see, because empty means unscoped, not empty.

The harness names the attached bases. Without that the model cannot tell "there
is nothing about this" from "I am only allowed to see one folder", and it
phrases a miss as the former.

**Sharing moves to the base.** A document is visible to whoever can see the base
it lives in, so `Document` is gone from the shareable types and
`documents.visible()` filters through `base_id`. "This folder is the team's" is
the granularity people think in; per-document grants meant answering "who can
see this?" by checking every file. Moving a document between bases changes who
can see it, so the destination has to be one you own.

`Document.base_id` is nullable only because the column had to be added to a
table that already had rows. `sweep_unfiled()` runs at startup beside the
orphaned-upload sweep and files anything predating bases into its owner's
default, which is what makes "always set" true everywhere else.

**The file input.** `.input` gave it a fixed height and horizontal padding, so
the browser's own button sat hard against the left edge while the filename
floated off the centre line. A file input is two controls in one box and
neither inherits anything useful, so it gets its own rule: no horizontal
padding, the button sized to `--control-h` with the divider that separates it,
and the text centred with line-height rather than flexbox, which file inputs do
not lay out reliably.

437 tests, ruff clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/lembas/api/chats.py

@@ -484,6 +484,26 @@ async def update_chat(request: Request, db: Db, user: RequiredUser, chat_id: str
             )
         chat.system_prompt = str(form["system_prompt"]).strip()[:8000]
 
+    if "knowledge_base_ids" in form:
+        # Sent as a single field even when empty, so that clearing every box
+        # actually clears the attachment -- absent checkboxes carry no signal of
+        # their own, which is the same trap update_chat exists to avoid.
+        from lembas.db.models import KnowledgeBase
+        from lembas.services.library import documents as documents_service
+
+        wanted = [value for value in form.getlist("knowledge_base_ids") if value]
+        chat.knowledge_bases = (
+            list(
+                db.scalars(
+                    documents_service.visible_bases(db, user).where(
+                        KnowledgeBase.id.in_(wanted)
+                    )
+                )
+            )
+            if wanted
+            else []
+        )
+
     submitted_params = {name: form[name] for name in _PARAM_RANGES if name in form}
     if submitted_params:
         if not allowed.get("chat.params"):