Fix fmd's broken Docker image and add missing cancel option to two installers

fmd.sh pointed at nulide/findmydevice, which no longer exists on Docker
Hub — the project has moved twice (nulide/findmydevice ->
gitlab.com/Nulide/findmydeviceserver -> gitlab.com/fmd-foss/fmd-server) and
was rewritten from Node.js to Go+React along the way, confirmed against the
current upstream repo and its GitLab container registry. This means the
service never actually started for anyone who installed it before this fix
("pull access denied for nulide/findmydevice, repository does not exist").

Switch to registry.gitlab.com/fmd-foss/fmd-server:0 (GitLab's own registry
has no "latest" tag; ":0" tracks the current major release the same way
this repo's other services use a floating tag). The old FMD_ADMIN_PASSWORD
model is gone from the app too — replaced with FMD_REGISTRATIONTOKEN
(self-registration gated by a token instead of one shared admin login), and
the database path moved from /fmd/data to /var/lib/fmd-server/db.

Also: filebrowser.sh and fmd.sh both showed "Manage that install (update /
full reinstall / cancel)" when re-run against an existing install, but
choosing "1) Manage" fell straight through into the same unconditional
fresh-install flow every time — no way to actually cancel or update in
place, contradicting both the banner text and the documented
prompt_reinstall_mode contract (CLAUDE.md's "Update vs. fresh reinstall on
rerun"). Wired both up to prompt_reinstall_mode, matching the reference
pattern in services/mattermost.sh. The same gap exists in 15 other
multi-instance services (vaultwarden, immich, audiobookshelf, homebox,
rustdesk, emby, meshcentral, traccar, lyrion, actualbudget, mealie, joplin,
jellyfin, unifi, ntfy) — not fixed here, flagged for a follow-up pass.

services/filebrowser.sh

@@ -242,6 +242,28 @@ install_filebrowser() {
             FB_DIR="$DOCKER_DIR/filebrowser-$_suffix"
             CONTAINER="filebrowser-$_suffix"
             log_info "New instance: $FB_DIR"
+        else
+            # "Manage that install" on THIS instance — the banner above promises
+            # update/fresh/cancel, so actually offer it instead of falling straight
+            # through into the same unconditional-overwrite flow as a new install.
+            if [[ -f "$FB_DIR/docker-compose.yml" ]]; then
+                local MODE=""
+                prompt_reinstall_mode MODE
+                case "$MODE" in
+                    update)
+                        log_info "Refreshing the FileBrowser image only — files-directory mount and Caddy config are left as-is."
+                        ( cd "$FB_DIR" && docker compose pull && docker compose up -d ) \
+                            && log_success "FileBrowser image refreshed" \
+                            || log_warning "Refresh failed — check: docker compose -f $FB_DIR/docker-compose.yml logs"
+                        return 0
+                        ;;
+                    cancel)
+                        log_info "Leaving the existing install as-is."
+                        return 0
+                        ;;
+                    fresh) ;;  # fall through to the full install flow below
+                esac
+            fi
         fi
     fi