feat: first-run setup wizard for client dropper config

- startup/client.lua now prompts on first run: asks if a dropper
  is next to the computer, and if so, which side it's on
- Saves config to .dropper_config (JSON with dropperSide,
  redstoneSide, enabled)
- Only launches dropperController if dropper is enabled
- dropperController.lua reads .dropper_config for its sides
  instead of hardcoding 'back'
- Delete .client_setup to re-run the setup wizard

dropperController.lua

@@ -1,10 +1,30 @@
--- Dropper Controller: Runs on Computer 1 (next to dropper_0)
--- Watches the dropper and pulses redstone until it's empty.
+-- Dropper Controller
+-- Watches a dropper peripheral and pulses redstone until it's empty.
 
-local REDSTONE_SIDE = "back"     -- side facing dropper_0
+local DROPPER_SIDE  = "back"     -- side where the dropper peripheral is
+local REDSTONE_SIDE = "back"     -- side facing dropper (for redstone output)
 local PULSE_TIME    = 0.3        -- redstone pulse duration in seconds
 local POLL_INTERVAL = 0.5        -- how often to check the dropper
-local DROPPER_SIDE  = "back"     -- side where the dropper is (as peripheral)
+
+-------------------------------------------------
+-- Load config from file if present
+-------------------------------------------------
+
+local _baseDir = fs.getDir(shell.getRunningProgram())
+local CONFIG_FILE = fs.combine(_baseDir, ".dropper_config")
+
+if fs.exists(CONFIG_FILE) then
+    local f = fs.open(CONFIG_FILE, "r")
+    local raw = f.readAll()
+    f.close()
+    local ok, cfg = pcall(textutils.unserialiseJSON, raw)
+    if ok and cfg then
+        if cfg.dropperSide  then DROPPER_SIDE  = cfg.dropperSide  end
+        if cfg.redstoneSide then REDSTONE_SIDE = cfg.redstoneSide end
+        if cfg.pulseTime    then PULSE_TIME    = cfg.pulseTime    end
+        if cfg.pollInterval then POLL_INTERVAL = cfg.pollInterval end
+    end
+end
 
 -------------------------------------------------
 -- Helpers