feat(previews): serve dynamic library previews

docs/library-switching.md

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+# Library Switching and Preview Routes
+
+Library switching no longer requires restarting the backend. The preview
+thumbnails shown in the UI are now served through a dynamic endpoint that
+resolves files against the folders registered for the active library at request
+time. This allows the multi-library flow to update model roots without touching
+the aiohttp router, so previews remain available immediately after a switch.
+
+## How the dynamic preview endpoint works
+
+* `config.get_preview_static_url()` now returns `/api/lm/previews?path=<encoded>`
+  for any preview path. The raw filesystem location is URL encoded so that it
+  can be passed through the query string without leaking directory structure in
+  the route itself.【F:py/config.py†L398-L404】
+* `PreviewRoutes` exposes the `/api/lm/previews` handler which validates the
+  decoded path against the directories registered for the current library. The
+  request is rejected if it falls outside those roots or if the file does not
+  exist.【F:py/routes/preview_routes.py†L5-L21】【F:py/routes/handlers/preview_handlers.py†L9-L48】
+* `Config` keeps an up-to-date cache of allowed preview roots. Every time a
+  library is applied the cache is rebuilt using the declared LoRA, checkpoint
+  and embedding directories (including symlink targets). The validation logic
+  checks preview requests against this cache.【F:py/config.py†L51-L68】【F:py/config.py†L180-L248】【F:py/config.py†L332-L346】
+
+Both the ComfyUI runtime (`LoraManager.add_routes`) and the standalone launcher
+(`StandaloneLoraManager.add_routes`) register the new preview routes instead of
+mounting a static directory per root. Switching libraries therefore works
+without restarting the application, and preview URLs generated before or after a
+switch continue to resolve correctly.【F:py/lora_manager.py†L21-L82】【F:standalone.py†L302-L315】