Show a progress bar and live log while Hammerhead, edges and sites update

The updater reports each step (checking, downloading with percent, verifying, installing, restarting).
Edges and sites send that progress and their log with their reports; Hammerhead serves it to a dialog
that opens when an update starts, and keeps its own last update log on disk across the restart.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

README.md

@@ -73,7 +73,7 @@ Releases are published for x86-64 only; for ARM machines build it yourself (`TAR
 
 Set `HAMMERHEAD_AUTO_UPDATE=1` (or pass `-auto-update`) and Hammerhead checks this server for a newer release every few hours, verifies it against `SHA256SUMS`, checks that the new binary runs, swaps it in and restarts itself. It only follows full releases (never pre-releases), never downgrades, and keeps the old binary if anything looks wrong. `setup.sh` turns it on. It is not available on Windows, and it is for the systemd and plain-binary installs: with Docker, pull a new image instead.
 
-Edges and sites report their version, so Server → Edges and Sites show which release each one runs and flag a newer one. An **Update** button appears for any machine that runs with auto-update on (`BLACKTIP_AUTO_UPDATE=1` / `BAMBOO_AUTO_UPDATE=1`): it asks that machine to install the newest release now, and the result is shown in the table. Under Server → Settings, **Check for updates** does the same lookup for Hammerhead itself, with an **Update now** button when auto-update is on. A machine without auto-update never installs anything on request.
+Edges and sites report their version, so Server → Edges and Sites show which release each one runs and flag a newer one. An **Update** button appears for any machine that runs with auto-update on (`BLACKTIP_AUTO_UPDATE=1` / `BAMBOO_AUTO_UPDATE=1`): it asks that machine to install the newest release now, and the result is shown in the table. Under Server → Settings, **Check for updates** does the same lookup for Hammerhead itself, with an **Update now** button when auto-update is on. A machine without auto-update never installs anything on request. Starting an update opens a progress bar with the live log of what the machine is doing (checking, downloading, verifying, installing, restarting). Hammerhead keeps the log of its own last update in `update.log` in its data folder, so it is still there after the restart.
 
 **With Docker**