Add a text widget (rich text: bold/italic/underline, per-run color/highlight)

A new self-contained widget type showing user-authored rich text -- no
live upstream to poll, like the static image widget, just word-wrapped
styled text instead of an uploaded image.

The dialog's contenteditable HTML is never stored or replayed as HTML:
app/text_content.py parses it server-side (on save) into a plain
paragraphs-of-styled-runs structure -- the actual sanitization
boundary, since raw HTML never round-trips back into any browser DOM
(the dialog rebuilds its editor from that same JSON via
createElement/textContent). app/widgets/text.py renders it with a
custom word-wrap/shrink-to-fit layout, using real vendored font weights
(app/fonts/NotoSans-{Regular,Bold,Italic,BoldItalic}.ttf, OFL-licensed
like the emoji fonts already there) rather than every other widget's
single ImageFont.load_default() -- the one widget type where that
distinction matters.

server/app/static/frame_layout.js

@@ -286,11 +286,11 @@ window.addEventListener('resize', () => {
 // icon was clicked).
 const DIALOG_INIT = {
   photos: initPhotosDialog, calendar: initCalendarDialog, whiteboard: initWhiteboardDialog,
-  tasks: initTasksDialog, static: initStaticDialog,
+  tasks: initTasksDialog, static: initStaticDialog, text: initTextDialog,
 };
 const DIALOG_CLOSE = {
   photos: closePhotosDialog, calendar: closeCalendarDialog, whiteboard: closeWhiteboardDialog,
-  tasks: closeTasksDialog, static: closeStaticDialog,
+  tasks: closeTasksDialog, static: closeStaticDialog, text: closeTextDialog,
 };
 
 let openDialogWidgetType = null;