v3.0.0

Release v3.0.0

Update all submodules to current releases, migrate to the new usage_cpp
API, make the standalone build link, and tidy the code.

- bump every submodule to its latest revision: bandwidth_cpp 3.0.0,
  cpu_cpp 1.3.3, kernel_cpp 1.1.0, throws_cpp 1.1.0, uptime_cpp 3.1.1,
  json_cpp 5.0.0, ubus_cpp 4.0.0, logger_cpp 2.3.2, usage_cpp 1.6.1 and
  common_cpp (the revision that builds on GCC 15).
- migrate main.cpp to the new usage_cpp API: the flat .name/.version/...
  fields move into the .info struct, usage.version_info() becomes
  usage.version() and usage.validate() (which threw) becomes the
  usage.validated check with usage.errors().
- the standalone Makefile never linked the system libraries: $(LIBS)
  (populated by ubus_cpp's Makefile.inc with -lubox -lblobmsg_json
  -lubus) was missing from the link line, and -luci was absent. Add both
  so systembus links outside the OpenWrt build environment.

Code fixes:
- ubus_cpu: `fail = "true"` assigned a string literal to a bool; use
  `fail = true`. The "key not supported" message printed res["key"]
  (empty) instead of the requested key.
- ubus_info_uptime: drop the shadowed inner key variable and its dead
  empty-key branch, flattening it into one if/else-if chain.

CI: build libubox, ubus and uci from source before make so the workflow
actually compiles on the runner, and refresh the actions (checkout@v5,
create/upload actions @v2, push/pull_request triggers, submodules in the
release checkout).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Signed-off-by: Oskari Rauta <oskari.rauta@gmail.com>