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/migration.py

@@ -511,6 +511,14 @@ def _migration_20(conn) -> None:
     Base.metadata.create_all(bind=conn)
 
 
+def _migration_21(conn) -> None:
+    """New widget type: a text widget shows user-authored rich text (see
+    app/text_content.py, app/widgets/text.py, models.TextWidgetConfig)
+    -- another no-live-upstream type like migration 20's static image.
+    Same brand-new-table create_all shape."""
+    Base.metadata.create_all(bind=conn)
+
+
 MIGRATIONS = [
     (1, _migration_1),
     (2, _migration_2),
@@ -532,6 +540,7 @@ MIGRATIONS = [
     (18, _migration_18),
     (19, _migration_19),
     (20, _migration_20),
+    (21, _migration_21),
 ]