add ability to include custom css & js in web interface (webmail, webaccount, webadmin), and use css variables in webmail for easier customization

if files {webmail,webaccount,webadmin}.{css,js} exist in the configdir (where
the mox.conf file lives), their contents are included in the web apps.

the webmail now uses css variables, mostly for colors. so you can write a
custom webmail.css that changes the variables, e.g.:

	:root {
		--color: blue
	}

you can also look at css class names and override their styles.

in the future, we may want to make some css variables configurable in the
per-user settings in the webmail. should reduce the number of variables first.

any custom javascript is loaded first. if it defines a global function
"moxBeforeDisplay", that is called each time a page loads (after
authentication) with the DOM element of the page content as parameter. the
webmail is a single persistent page. this can be used to make some changes to
the DOM, e.g. inserting some elements. we'll have to see how well this works in
practice. perhaps some patterns emerge (e.g. adding a logo), and we can make
those use-cases easier to achieve.

helps partially with issue #114, and based on questions from laura-lilly on
matrix.

mox-/webappfile.go

@@ -7,6 +7,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"io/fs"
 	"log/slog"
 	"net/http"
 	"os"
@@ -25,6 +26,7 @@ import (
 type WebappFile struct {
 	HTML, JS         []byte // Embedded html/js data.
 	HTMLPath, JSPath string // Paths to load html/js from during development.
+	CustomStem       string // For trying to read css/js customizations from $configdir/$stem.{css,js}.
 
 	sync.Mutex
 	combined     []byte
@@ -107,28 +109,82 @@ func (a *WebappFile) Serve(ctx context.Context, log mlog.Log, w http.ResponseWri
 		}
 	}
 
+	// Check mtime of css/js files.
+	var haveCustomCSS, haveCustomJS bool
+	checkCustomMtime := func(ext string, have *bool) bool {
+		path := ConfigDirPath(a.CustomStem + "." + ext)
+		if fi, err := os.Stat(path); err != nil {
+			if !errors.Is(err, fs.ErrNotExist) {
+				a.serverError(log, w, err, "stat customization file")
+				return false
+			}
+		} else if mtm := fi.ModTime(); mtm.After(diskmtime) {
+			diskmtime = mtm
+			*have = true
+		}
+		return true
+	}
+	if !checkCustomMtime("css", &haveCustomCSS) || !checkCustomMtime("js", &haveCustomJS) {
+		return
+	}
+	// Detect removal of custom files.
+	if fi, err := os.Stat(ConfigDirPath(".")); err == nil && fi.ModTime().After(diskmtime) {
+		diskmtime = fi.ModTime()
+	}
+
+	a.Lock()
+	refreshdisk = refreshdisk || diskmtime.After(a.mtime)
+	a.Unlock()
+
 	gz := AcceptsGzip(r)
 	var out []byte
 	var mtime time.Time
 	var origSize int64
 
-	func() {
+	ok := func() bool {
 		a.Lock()
 		defer a.Unlock()
 
 		if refreshdisk || a.combined == nil {
-			script := []byte(`<script>/* placeholder */</script>`)
-			index := bytes.Index(html, script)
-			if index < 0 {
-				a.serverError(log, w, errors.New("script not found"), "generating combined html")
-				return
+			var customCSS, customJS []byte
+			var err error
+			if haveCustomCSS {
+				customCSS, err = os.ReadFile(ConfigDirPath(a.CustomStem + ".css"))
+				if err != nil {
+					a.serverError(log, w, err, "read custom css file")
+					return false
+				}
+			}
+			if haveCustomJS {
+				customJS, err = os.ReadFile(ConfigDirPath(a.CustomStem + ".js"))
+				if err != nil {
+					a.serverError(log, w, err, "read custom js file")
+					return false
+				}
+			}
+
+			cssp := []byte(`/* css placeholder */`)
+			cssi := bytes.Index(html, cssp)
+			if cssi < 0 {
+				a.serverError(log, w, errors.New("css placeholder not found"), "generating combined html")
+				return false
+			}
+			jsp := []byte(`/* js placeholder */`)
+			jsi := bytes.Index(html, jsp)
+			if jsi < 0 {
+				a.serverError(log, w, errors.New("js placeholder not found"), "generating combined html")
+				return false
 			}
 			var b bytes.Buffer
-			b.Write(html[:index])
-			fmt.Fprintf(&b, "<script>\n// Javascript is generated from typescript, don't modify the javascript because changes will be lost.\nconst moxversion = \"%s\";\nconst moxgoos = \"%s\";\nconst moxgoarch = \"%s\";\n", moxvar.Version, runtime.GOOS, runtime.GOARCH)
+			b.Write(html[:cssi])
+			fmt.Fprintf(&b, "/* Custom CSS by admin from $configdir/%s.css: */\n", a.CustomStem)
+			b.Write(customCSS)
+			b.Write(html[cssi+len(cssp) : jsi])
+			fmt.Fprintf(&b, "// Custom JS by admin from $configdir/%s.js:\n", a.CustomStem)
+			b.Write(customJS)
+			fmt.Fprintf(&b, "\n// Javascript is generated from typescript, don't modify the javascript because changes will be lost.\nconst moxversion = \"%s\";\nconst moxgoos = \"%s\";\nconst moxgoarch = \"%s\";\n", moxvar.Version, runtime.GOOS, runtime.GOARCH)
 			b.Write(js)
-			b.WriteString("\t\t</script>")
-			b.Write(html[index+len(script):])
+			b.Write(html[jsi+len(jsp):])
 			out = b.Bytes()
 			a.combined = out
 			if refreshdisk {
@@ -152,7 +208,7 @@ func (a *WebappFile) Serve(ctx context.Context, log mlog.Log, w http.ResponseWri
 				}
 				if err != nil {
 					a.serverError(log, w, err, "gzipping combined html")
-					return
+					return false
 				}
 				a.combinedGzip = b.Bytes()
 			}
@@ -160,7 +216,11 @@ func (a *WebappFile) Serve(ctx context.Context, log mlog.Log, w http.ResponseWri
 			out = a.combinedGzip
 		}
 		mtime = a.mtime
+		return true
 	}()
+	if !ok {
+		return
+	}
 
 	w.Header().Set("Content-Type", "text/html; charset=utf-8")
 	http.ServeContent(gzipInjector{w, gz, origSize}, r, "", mtime, bytes.NewReader(out))