docs: Add RPCD and LuCI frontend guidelines to CLAUDE.md

Document lessons learned from WireGuard dashboard development:
- jshn argument size limits and manual JSON workaround
- UCI underscore-prefixed private data storage
- LuCI RPC expect field unwrapping behavior
- LuCI JS module caching and cache-busting strategies
- Quick deploy workflow for JS/RPCD changes
- Common pitfalls (params ordering, sessionStorage, interface naming)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

CLAUDE.md

@@ -108,3 +108,64 @@ ssh root@192.168.255.1 '/etc/init.d/rpcd restart'
   ```
 
 - If unsure, check `OPENWRT_ONLY_PACKAGES` in `secubox-tools/local-build.sh`
+
+## RPCD Backend Scripting (Shell-based RPC handlers)
+
+### jshn Argument Size Limits
+- **`json_add_string` cannot handle large values** (e.g., base64-encoded images/SVGs)
+- jshn passes values as shell arguments, which hit BusyBox's argument size limit ("Argument list too long")
+- **Workaround**: Build JSON output manually via file I/O instead of jshn:
+  ```sh
+  local tmpfile="/tmp/wg_output_$$.json"
+  printf '{"field":"' > "$tmpfile"
+  # Stream large data via pipe/redirect (never as argument)
+  some_command | base64 -w 0 >> "$tmpfile"
+  printf '"}\n' >> "$tmpfile"
+  cat "$tmpfile"
+  rm -f "$tmpfile"
+  ```
+- This applies to any RPCD method that returns large blobs (QR codes, certificates, etc.)
+
+### UCI Private Data Storage
+- Use underscore-prefixed option names for internal/hidden data: `uci set network.section._private_field="value"`
+- These are not shown in standard LuCI forms but are accessible via `uci -q get`
+- Useful for storing client private keys, internal state, etc.
+
+## LuCI JavaScript Frontend
+
+### RPC `expect` Field Behavior
+- **`rpc.declare({ expect: { field: '' } })` unwraps the response** — it returns ONLY the value of `field`, not the full object
+- If the backend returns `{"config": "...", "error": "..."}` and expect is `{ config: '' }`, the result is just the config string — `result.error` is undefined
+- **Use `expect: { }` (empty object) when you need the full response** including error fields
+- Use `expect: { field: default }` only when you always want just that one field and don't need error handling
+
+### Module Caching
+- **LuCI's JS module loader caches parsed modules in memory** — `Ctrl+Shift+R` does NOT clear this
+- Clearing browser cache, `rm /tmp/luci-indexcache*`, and `rm /tmp/luci-modulecache/*` may not be enough
+- **Reliable fix**: Force full page navigation with cache-busting query param:
+  ```js
+  window.location.href = window.location.pathname + '?' + Date.now();
+  ```
+- For development, set `uci set uhttpd.main.no_cache=1 && uci commit uhttpd && /etc/init.d/uhttpd restart`
+
+### Quick Deploy for LuCI JS/RPCD Changes
+- LuCI JS views and shared resources can be deployed directly to the router without rebuilding:
+  ```bash
+  # Deploy JS views
+  scp htdocs/luci-static/resources/view/<app>/*.js root@192.168.255.1:/www/luci-static/resources/view/<app>/
+
+  # Deploy shared JS libraries
+  scp htdocs/luci-static/resources/<app>/*.js root@192.168.255.1:/www/luci-static/resources/<app>/
+
+  # Deploy RPCD handler and restart
+  scp root/usr/libexec/rpcd/<handler> root@192.168.255.1:/usr/libexec/rpcd/
+  ssh root@192.168.255.1 '/etc/init.d/rpcd restart'
+
+  # Clear LuCI caches on router
+  ssh root@192.168.255.1 'rm -f /tmp/luci-indexcache* /tmp/luci-modulecache/*'
+  ```
+
+### Common Pitfalls
+- **RPC params order matters**: The `params` array in `rpc.declare()` must match the positional arguments in `addPeer(arg1, arg2, ...)` calls — adding a new param means updating ALL callers
+- **sessionStorage is volatile**: Data stored in `sessionStorage` is lost on tab close/refresh — don't rely on it for persistent data; use UCI backend storage instead
+- **Interface name conflicts**: When creating WireGuard interfaces, always check for existing names (wg0, wg1, etc.) and auto-increment to the next available name