feat: add canvas annotation system (rect, line, text draw tools)

[theblondealex]

Why

Working on a large project in Collab often means dozens of terminal and browser tiles scattered across a canvas with no way to communicate structure to yourself have order to the different projects you might run in parralell. There's no way to group related tiles, label what something does, or draw a connection between two terminals that are part of the same pipeline.

This comes up directly in #29 (canvas needs a way to visually group/organise tiles) and #25 (users want to annotate and label their workspace). Right now the only option is careful spatial arrangement, which breaks down fast as a canvas grows.

This PR adds a lightweight annotation layer — rects to group tiles, lines to connect them, and text labels — so you can impose structure on a canvas without changing how tiles themselves work.

Demo video

annotations.1.1.mov

note since recording the video the location of the draw has moved to near the + like below, and colour of the popovers to match DS

Changes

File	What changed
src/windows/shell/src/annotation-manager.ts	New — pure state module: types, add/remove/update, selection, persist/restore
src/windows/shell/src/annotation-renderer.ts	New — all SVG/DOM rendering, draw interactions, drag, popover, mode management
src/windows/shell/index.html	Annotation layer DOM, draw toolbar button + tool popover
src/windows/shell/src/shell.css	Annotation styles: layers, popovers, tile highlight glows, selection ring
src/windows/shell/src/renderer.js	Wires annotation renderer into canvas; group drag + delete + restore
src/windows/shell/src/tile-interactions.js	onGroupDragStart / onGroupDragMove / onGroupDragEnd callbacks for cross-system drag
src/windows/shell/src/tile-manager.js	Threads group drag callbacks through to attachDrag
src/main/canvas-persistence.ts	Adds AnnotationState union type to the persisted canvas schema

How it works

• Annotations render behind the tile layer: SVG <g> elements for rects and lines (inside a masked group that hides annotations under tiles), DOM <div> elements for text
• All three tools are single-click/drag to create, then revert to pointer mode automatically
• Lines can be free (canvas coords) or connected (anchored to two tile centers — they track tile movement and only detach when dragged individually)
• Multi-select marquee works across tiles and annotations together; dragging a mixed selection moves everything as a group
• Annotations persist to canvas-state.json alongside tiles on every change; annotations is optional in the schema so existing state files load cleanly
• Default colour is derived from --muted CSS variable so it adapts to light/dark mode

Closes #29, closes #25