gui: Use localised time in duration (#9552)

https://github.com/syncthing/syncthing/pull/8291 inpired me to develop
this. I tested it with all the languages Syncthing currently supports,
and they all work.

The only issue is that when you change the language in the GUI, you have
to either refresh the page, or wait a few seconds for the page to
refresh by itself, before the duration is translated into the new
language.

### Screenshots


![2024-05-20_21-47-58](https://github.com/syncthing/syncthing/assets/220772/7e3b371e-3495-4e3e-853a-b5a41215e6c7)

### Documentation

The documentation for the translation widget is at
https://github.com/EvanHahn/HumanizeDuration.js/blob/main/README.md

gui/default/syncthing/core/durationFilter.js

@@ -7,16 +7,63 @@
  * {{1|duration:"h"}}         --> <1h
 **/
 angular.module('syncthing.core')
-    .filter('duration', function () {
+    .filter('duration', function ($translate) {
         'use strict';
 
         var SECONDS_IN = { "d": 86400, "h": 3600, "m": 60, "s": 1 };
         return function (input, precision) {
-            var result = "";
             if (!precision) {
                 precision = "s";
             }
             input = parseInt(input, 10);
+            var language_cc = $translate.use();
+            if (language_cc != null) {
+                language_cc = language_cc.replace("-", "_");
+                var fallbacks = [];
+                var language = language_cc.substr(0, 2);
+                switch (language) {
+                case "zh":
+                    // Use zh_TW for zh_HK
+                    fallbacks.push("zh_TW");
+                    break
+                }
+                if (language != language_cc) {
+                    fallbacks.push(language);
+                }
+                // Fallback to english, if the language isn't found
+                fallbacks.push("en");
+
+                var units = ["d", "h", "m", "s"];
+                switch (precision) {
+                    case "d":
+                        units.pop();
+                        // fallthrough
+                    case "h":
+                        units.pop();
+                        // fallthrough
+                    case "m":
+                        units.pop();
+                        // fallthrough
+                    case "s":
+                        break
+                    default:
+                        return "[Error: precision must be d, h, m or s, it's " + precision + "]";
+                }
+
+                try {
+                    // humanizeDuration accepts only milliseconds
+                    return humanizeDuration(input * 1000, {
+                        language: language_cc,
+                        maxDecimalPoints: 0,
+                        units: units,
+                        fallbacks: fallbacks
+                    });
+                } catch(err) {
+                    console.log(err.message + ": language_cc=" + language_cc)
+                    // if we crash, fallthrough to english
+                }
+            }
+            var result = "";
             for (var k in SECONDS_IN) {
                 var t = (input / SECONDS_IN[k] | 0); // Math.floor