Add frames: named regions that group and carry their shapes

[raj-khan]

FL-83, part 2 of 2.

• Frame shape in the library (560x360 by default): a dashed region behind everything, named at its top left. Membership is where things sit: a shape belongs to the smallest frame its centre is in, so there is no list to keep in step, and frames nest. A click never opens its details, so the shapes inside stay easy to reach.
• Dragging a frame carries its members, saved as one change (one undo).
• Frame menu (right click or long press): Rename, Copy frame for AI (a brief of just that frame), Export frame (Export gains a what to export select), Delete frame.
• Everywhere else: the brief gets a Frames section; Mermaid draws nested subgraphs; draw.io exports frames first and round-trips them; the SVG renderer draws them under the connections; .flow is id = frame "Name", documented in /docs/format and llms-full.txt.

Tests: e2e/frames.spec.js (5), src/domain/__tests__/frames.spec.js (9). e2e 136, vitest 270, lint and typecheck; checked by screenshot.