fix: stop the file-viewer reload loop on watched files

The watch callback forwarded every FS event, including the pure reads the
viewer's own GET /api/file produces (IN_ACCESS / IN_CLOSE_NOWRITE on Linux):
each silent reload re-triggered the watcher, looping at ~1 Hz. For PDFs every
iteration minted a new blob URL and re-assigned iframe.src, which re-runs
Chrome's whole PDF viewer (the flicker) and pushes a joint session-history
entry (the back button buried under hundreds of blob: entries).

- file_watch: forward an event only when the content version (mtime_ns, len)
  actually moved; drop Access events outright, stat-compare the rest.
- viewer: render pdf/latex/svg previews in a keyed() iframe — a fresh
  element's first navigation replaces its history slot instead of pushing.

web/index.html

@@ -78,6 +78,7 @@
     "imports": {
       "lit":                           "/vendor/lit-all.min.js",
       "lit/directives/unsafe-html.js": "/vendor/lit-all.min.js",
+      "lit/directives/keyed.js":       "/vendor/lit-all.min.js",
       "marked":                        "/vendor/marked.esm.js",
       "dompurify":                     "/vendor/purify.es.mjs",
       "cronstrue":                     "/vendor/cronstrue.mjs"