release prep: opt-in web UI, make uninstall, fix stale kernel-catalog docs

- WebListenAddr default is now "" (empty). The experimental web UI was
  running on 127.0.0.1:7777 by default, which surprises users who never
  opted in. Users who want it set `web_listen_addr = "127.0.0.1:7777"`
  in config.toml.
- `make uninstall` stops the daemon (if any) and removes the installed
  binaries. Preserves user data on disk but prints the paths so `rm -rf`
  can follow for a full purge. Documented in README next to install.
- docs/kernel-catalog.md: replace the `void-6.12` and `alpine-3.23`
  examples (never published) with `generic-6.12` (the only cataloged
  kernel today). Updates the versioning-convention example too.

README.md

@@ -32,6 +32,19 @@ Installs `banger` (CLI), `bangerd` (daemon, auto-starts on first
 CLI call), and `banger-vsock-agent` (companion, under
 `$PREFIX/lib/banger/`).
 
+To remove the binaries (and stop the daemon):
+
+```bash
+make uninstall
+```
+
+User data stays in place — the target prints the paths so you can
+`rm -rf` them if you want a full purge:
+
+- `~/.config/banger/` — config, managed SSH keys
+- `~/.local/state/banger/` — VM records, rootfs images, kernels, daemon DB/log
+- `~/.cache/banger/` — OCI layer cache
+
 ### Shell completion
 
 `banger` ships completion scripts for bash, zsh, fish, and
@@ -116,8 +129,8 @@ Most commonly set:
 - `ssh_key_path` — host SSH key. If unset, banger creates
   `~/.config/banger/ssh/id_ed25519`.
 - `firecracker_bin` — override the auto-resolved `PATH` lookup.
-- `web_listen_addr` — experimental web UI (default `127.0.0.1:7777`;
-  set to `""` to disable).
+- `web_listen_addr` — experimental web UI; disabled by default. Set
+  e.g. `"127.0.0.1:7777"` to enable.
 
 Full key list in `internal/config/config.go`.