fix(erd): interface review polish (notes, keys, views menu, grouping hint, marks)

[ZhuchkaTriplesix]

Items 5–9 of the ER diagram interface review (1, 2, 4 were #1299, 3 was #1300).

• Plain note: a note without a colour was filled with the canvas colour and had only a hairline. It is now a slightly darker sheet (7 % of the muted foreground) with the cards' shadow; the SVG and PDF use the same tint.
• Keys after a note: touching a note, its resize handle or a group title takes the keyboard back to the canvas; before, Esc / Ctrl+0 / Ctrl+F did nothing if the focus had been elsewhere (another pane, a closed dialog).
• Views menu: every row has a radio mark (the active one filled), so labels line up; names over 28 characters are cut with an ellipsis in the menu and on the toolbar.
• Grouping hint: a card's New group… entry shows Shift+click marks more.
• Marked card: a ring outside the card instead of a 2.5 px border, which took 3 px from the content width the card is measured for and could cut a long type.

Tests: test/features/erd/erd_ui_review_polish_test.dart — note fill and shadow; Ctrl+0 works after tapping a note with the focus on a widget outside the diagram; the long view name; the hint; the marked card's border width and ring.