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/tests/test_migrations.py

@@ -76,6 +76,7 @@ def test_expected_columns_exist_on_current_schema():
     assert "calendar_week_start_offset" in frame_columns
     assert "name" in task_widget_columns  # migration 19
     assert "static_widget_configs" in inspector.get_table_names()  # migration 20
+    assert "text_widget_configs" in inspector.get_table_names()  # migration 21
 
 
 # --- widget system backfill (migration 16 + _ensure_widgets_backfilled) ---