web: move all strings to i18n & improve a11y

- omnibox is now fully usable with a screen reader
- back button is now interpreted as such
- subtext now accepts line breaks

web/i18n/en/a11y/save.json

@@ -1,3 +1,9 @@
 {
-    "linkArea": "link input area"
+    "linkArea": "link input area",
+    "clearInput": "clear input",
+    "download": "download",
+    "downloadThink": "processing the link...",
+    "downloadCheck": "verifying download...",
+    "downloadDone": "downloading done",
+    "downloadError": "downloading error"
 }