fix: RS485 transparent pass-through + LuCI dark mode fixes

[weicheng04]

Summary

Two independent fixes bundled in one PR (no file overlap between them).

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1. RS485-MQTT transparent bridge (rs485-module, 5 files)

Problem: The previous implementation was not actually transparent — it broke binary data in three ways:

• Uplink wrapped bytes in a JSON envelope {"data":"..."}, breaking Modbus RTU and other binary protocols.
• String::from_utf8_lossy() replaced non-UTF-8 bytes with U+FFFD, and .trim() stripped whitespace from raw frames.
• Downlink rejected any payload not shaped as {"data":"..."}, preventing binary clients from sending raw bytes.

Fix:

• main.rs: publish / write_all raw byte slices directly, no JSON wrap.
• Replace serde + serde_json deps with hex (used for log readability only).
• Fix misleading ConnAck log ("Published [RS485->MQTT]" was logged on connect but nothing was actually published → changed to "MQTT connected, uplink ready on").
• uci-defaults/73_rs485-module: rewrite to populate port1/port2/port3 per-port sections (previous code wrote flat mqtt.* / serial.* sections that nothing reads).
• rs485-module.init: drop stale serial.enabled gate, start one transparent instance per port unless protocol_enabled=1.

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2. LuCI dark mode fixes (luci-theme-sensecap + 6 luci-app-* JS, 11 files)

Problem: SenseCAP app JS injected hardcoded inline colors (background:#fff, color:#333, border:#ccc, etc.) in fixed-height log viewers, chart containers, and a Basic Station upload dropzone. Inline styles have CSS specificity 1000, so
dark.css selectors could not override them — leaving "white holes" when the browser switched to dark mode.

Fix:

• Add 13 --sensecap-* CSS variables (light defaults in cascade.css/.less + dark overrides in dark.css/.less): log viewer bg/fg/border, chart bg/border, muted bg, dropzone border/hover-bg, muted/emphasis fg, divider, neutral button bg/hover.
• Replace hardcoded colors in 7 JS files with var() references:
  • luci-app-rs485/.../rs485/log.js
  • luci-app-lora/.../lora/log.js
  • luci-app-lora/.../status/include/19_lora.js (chart container)
  • luci-app-lora/.../lora-platform/basicstation.js (dropzone stylesheet)
  • luci-app-ups/.../ups/log.js, ups/ups.js
  • luci-app-ota/.../system/ota.js
• Basic Station .sfu-* stylesheet: brand green #8FC320 → reuse var(--primary).

Note: cascade.css / dark.css are LESS compilation outputs (Easy LESS VS Code plugin). Both .less source and .css artifact were updated in lockstep — the OpenWrt Makefile does not invoke lessc, so editing .less alone would have no
effect on the deployed artifact.

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Test plan

• Compile passes locally (openwrt-24.10, full make -j28)
• All ipk installed on test container 10.0.0.160 via opkg install --force-reinstall
• logread | grep RS485-MQTT shows new log line "MQTT connected, uplink ready on" (old build printed "Published [RS485->MQTT]")
• CSS variables verified present in /www/luci-static/argon/css/{cascade,dark}.css on the container
• var(--sensecap-*) references verified present in 6/7 JS files on the container
• RS485 binary pass-through end-to-end test with real Modbus device (pending hardware)
• Dark mode visual check on all 6 affected pages (in progress)

Known follow-ups (not in this PR)

• luci-app-rs485 was skipped during deployment because the container had a newer luci feed version (26.146.42521~e3667f0 > our build 26.146.05840~441152d). The fix is in the source tree and will take effect after the next build with
  refreshed luci feed. As a workaround, rs485/log.js can be scp'd directly to /www/luci-static/resources/view/rs485/log.js.
• chirpstack v4.15.0 main package fails to build due to an upstream rquickjs-sys 0.9.0 + system clang incompatibility (__gnuc_va_list not found); unrelated to this PR.
• luci-theme-sensecap and luci-app-gateway both ship /usr/share/ucode/luci/template/themes/argon/sysauth.ut — packaging bug to be reported upstream.