GeezerVison

Adds the Storybook stories for the Coder Agents Projects page and updates the user docs for projects, pinning and the new page. It is stacked on the page PR and split out from it to keep that PR under the size limit.

• Stories: ProjectsPageView.stories.tsx covers the populated grid on each tab, loading, the error state, both empty states, the focused unpin button, the open actions menu and a phone-sized viewport, so Pixel screenshots catch visual regressions.
• Docs: docs/coder-agents/projects.md now describes the Projects page and its two tabs, how sharing works (and what happens when chat sharing is disabled), pinning and unpinning from cards and the sidebar, reordering pinned projects, which chats the sidebar lists (chats in unpinned projects show only if pinned themselves; chats in a project you can no longer open stay listed), and the dialog shown when a chat moves into an unpinned project.