[openwrt] remove the in-tree LuCI app (proposed for openwrt/luci)

[LorbusChris]

Summary

Removes the in-tree OpenWrt LuCI app from ot-br-posix, so the web interface can be
maintained in the openwrt/luci repository instead (luci-app-openthread),
following the standard convention for LuCI apps, as requested by the OpenWrt
maintainers (#3431, openwrt/packages#29791).

Deletes the legacy Lua controller, the admin_thread .htm templates and
handle_error.js, stops installing them from the OpenWrt package, and drops the
now-unused luci-lua-runtime dependency.

Context

This PR previously rewrote the in-tree app to client-side JS + ucode. The OpenWrt
maintainers asked to move the app to openwrt/luci instead (#3431) rather than keep
it here, so this PR now just removes it. The proposed replacement app and the feed
change are open in parallel:

• luci-app-openthread: add new application openwrt/luci#8871 (open) — proposes adding the app to openwrt/luci, rewritten
  for the modern client-side LuCI framework. It talks to otbr-agent's existing ubus
  API and works against the current release; no ot-br-posix change is required for it.
• openthread-br: drop luci-app-openthread, proposed for luci repo openwrt/packages#30096 (open) — proposes dropping the in-package
  luci-app-openthread from the openthread-br feed package.

These are all still proposals under review; this removal should land in coordination
with them so users aren't left without the app.

An earlier revision of this PR also normalized the otbr ubus reply field names to
snake_case. That was dropped: it was incomplete/error-prone (the rename collided
with method names in the ubus dispatch table, e.g. leaderdata/joinernum), and
the openwrt/luci app simply reads the daemon's existing field names instead, which
removes the cross-repo ordering dependency entirely.

Refs: #3431, openwrt/packages#29791, openwrt/luci#8871

[LorbusChris]

cc @stintel

[gemini-code-assist]

Code Review

This pull request migrates the OpenThread Border Router LuCI application from the legacy Lua-based framework to the modern LuCI client-side JavaScript framework and ucode backend. The changes involve replacing Lua controllers and templates with JS views and an RPCD ucode script. Review feedback highlights a logic error in the signal icon mapping and suggests strengthening input validation for the network key and PSKc fields by enforcing hexadecimal character checks.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 36.87%. Comparing base (2b41187) to head (4b19ef3).
⚠️ Report is 1597 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #3326       +/-   ##
===========================================
- Coverage   55.77%   36.87%   -18.90%     
===========================================
  Files          87      144       +57     
  Lines        6890    17507    +10617     
  Branches        0     1431     +1431     
===========================================
+ Hits         3843     6456     +2613     
- Misses       3047    10683     +7636     
- Partials        0      368      +368     

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[jwhui]

Thanks! 👍🏼