fipstop: add "Listening on fips0" panel to Node tab

Surfaces local services reachable from the mesh, paired with their
current `inet fips` baseline filter classification. Lands to the
right of the existing TUN section in the Traffic block.

A new daemon control query `show_listening_sockets` returns IPv6
listeners bound to either `::` (wildcard) or the node's fd00::/8
address, each classified as Accept / Drop / Unknown / NoFirewall
against the running inbound chain. fipstop renders the result as a
table beside the Traffic counters: Accept rows in default White,
Drop / Unknown in DarkGray, a yellow banner above the table when
`fips-firewall.service` is inactive, and a trailing `*` on
wildcard binds to remind the operator the bind is not
fips0-specific.

Daemon side:

- `src/control/listening.rs` walks `/proc/net/tcp6` and
  `/proc/net/udp6` via the procfs crate (LISTEN state for TCP,
  wildcard remote for UDP), filters to fips0-reachable binds, and
  resolves inodes to PID / comm via `/proc/<pid>/fd`.

- `src/control/firewall_state.rs` shells out to
  `nft -j list table inet fips` and walks the inbound chain.
  Recognises canonical accepts (`tcp/udp dport N accept`,
  `dport { ... } accept`, `dport A-B accept`), the iifname-scoping
  line, conntrack and icmpv6 lines (skipped). Any rule with
  unrecognised matchers (saddr filters, jumps, daddr filters) or
  non-terminal verdicts forces Unknown classification for the
  ports it references. Eleven unit tests cover the classification
  logic; the listening enumerator carries a /proc-parsing test of
  its own.

- `show_listening_sockets` emits
  `{fips0_addr, firewall_active, sockets[]}` with per-row
  `{proto, local_addr, port, pid, process, filter, wildcard_bind}`.

fipstop side:

- `src/bin/fipstop/ui/dashboard.rs` splits the Traffic block into
  a 50/50 horizontal layout; the existing TUN + Forwarded panel
  occupies the left half.

- `src/bin/fipstop/ui/listening.rs` renders the right half.

- `main.rs` fetches the new query each tick when the Node tab is
  active. Errors are non-fatal: an old daemon without the query
  leaves the payload at None and the panel renders "loading...".

`Cargo.toml` gains `procfs = "0.18"` on the Linux target. IPv4
listeners are not enumerated — fips0 is IPv6-only.

Folded in: revert the default-socket lookup from writability-probe
back to existence-based selection. The previous tempfile-probe on
`/run/fips` silently steered fipstop / fipsctl onto an XDG path
the daemon never bound for any user in the `fips` group whose
shell session had not yet picked up the supplementary group (no
re-login after `usermod -aG`). `XDG_RUNTIME_DIR` is set on every
modern systemd-managed user session, so this hit the common case.
The kernel checks actual group membership at `connect(2)`, so a
user who genuinely cannot connect now gets a clear `EACCES`
rather than a silent path mismatch. Drops the now-unused
`is_writable_dir` helper. `XDG_RUNTIME_DIR` existence validation
is preserved.

Documentation:

- `docs/reference/cli-fipstop.md` — Node-tab row updated, new
  "Listening on fips0 panel" section.
- `docs/reference/control-socket.md` — `show_listening_sockets`
  added to the read-only queries table.
- `docs/how-to/enable-mesh-firewall.md` — new "Verify with
  fipstop" section.
- `docs/tutorials/host-a-service.md` — fipstop callouts at
  Steps 3, 5, 6 + Troubleshooting bullet + wildcard-bind reminder
  under "What you've learned".
- `CHANGELOG.md` — new bullet under `Added / Operator Tooling`,
  resolver `Fixed` entry rewritten to describe the
  existence-based final shape.

Cargo.toml

@@ -42,6 +42,7 @@ libc = "0.2"
 [target.'cfg(target_os = "linux")'.dependencies]
 rtnetlink = "0.21.0"
 rustables = "0.8.7"
+procfs = { version = "0.18", default-features = false }
 
 # bluer/BlueZ needs glibc — see build.rs `bluer_available` cfg gate.
 [target.'cfg(all(target_os = "linux", not(target_env = "musl")))'.dependencies]