feat(dev): add Air config and Makefile dev target

Add .air.toml for live reload development with Air. Update Makefile to
include a 'dev' target that starts Air with custom cache directories.
Improve .gitignore to exclude 'tmp' directory used by Air.

Refactor frontend JS to use querySelector for robustness. Update HTML
template to fix form method, script loading, and message div syntax.
c

.air.toml

@@ -0,0 +1,52 @@
+root = "."
+testdata_dir = "testdata"
+tmp_dir = "tmp"
+
+[build]
+args_bin = []
+bin = "./tmp/main"
+cmd = "go build -o ./tmp/main ./cmd/payit/"
+delay = 1000
+exclude_dir = ["assets", "tmp", "vendor", "testdata"]
+exclude_file = []
+exclude_regex = ["_test.go"]
+exclude_unchanged = false
+follow_symlink = false
+full_bin = ""
+include_dir = []
+include_ext = ["go", "tpl", "tmpl", "html"]
+include_file = []
+kill_delay = "0s"
+log = "build-errors.log"
+poll = false
+poll_interval = 0
+post_cmd = []
+pre_cmd = []
+rerun = false
+rerun_delay = 500
+send_interrupt = false
+stop_on_error = false
+
+[color]
+app = ""
+build = "yellow"
+main = "magenta"
+runner = "green"
+watcher = "cyan"
+
+[log]
+main_only = false
+silent = false
+time = false
+
+[misc]
+clean_on_exit = false
+
+[proxy]
+app_port = 0
+enabled = false
+proxy_port = 0
+
+[screen]
+clear_on_rebuild = false
+keep_scroll = true

.gitignore

@@ -1,2 +1,3 @@
 *cache
 *env*
+tmp

Makefile

@@ -4,7 +4,11 @@ GOCACHE ?= $(CURDIR)/.cache
 GOMODCACHE ?= $(CURDIR)/.modcache
 GOFLAGS ?= -count=1
 
-.PHONY: fmt lint test clean tidy
+.PHONY: dev fmt lint test clean tidy
+
+dev:
+	@echo "Starting development environment"
+	@GOCACHE=$(GOCACHE) GOMODCACHE=$(GOMODCACHE) air -c .air.toml
 
 fmt:
 	@echo "Formatting Go files"