fix(opensidian): unify content model and modernize UI components

[braden-w]

Opensidian's content editor was silently writing to a store nothing else reads. Meanwhile, the UI was built with raw HTML inputs and inline SVGs instead of shared components. This PR fixes both.

Content model unification

Document Y.Docs had two independent content stores—Y.Text('content') (used by handle.read()/write()) and Y.Array('timeline') (used by fs.writeFile()/readFile()). Content written through the editor was invisible to the filesystem API and vice versa.

// BEFORE: editor writes to Y.Text('content'), fs reads from Y.Array('timeline')
const handle = await ws.documents.files.content.open(id);
handle.read();          // → reads Y.Text('content') — stale/empty
handle.write('hello');  // → writes Y.Text('content') — invisible to fs

// AFTER: editor uses the same timeline store as the filesystem
await fs.content.read(id);        // → reads from timeline
await fs.content.write(id, data); // → writes to timeline

This is Phase 1 of the unification spec (specs/20260313T224500-unify-document-content-model.md). Phase 2 will promote the timeline into packages/workspace so handle.read()/write() itself becomes timeline-backed—making both paths equivalent.

Documentation updated across skills, READMEs, and JSDoc to mark handle.read()/write() and handle.ydoc.getText('content') as anti-patterns until Phase 2 lands.

UI modernization

Replaced raw HTML and custom components with shared @epicenter/ui primitives and shadcn-svelte-extras:

Before                              After
─────────────────────────────────   ──────────────────────────────────
Raw <input>, <button>               Input, Button from @epicenter/ui
Inline SVG icons                    lucide-svelte icons
Custom TreeNode.svelte (177 lines)  tree-view from shadcn-svelte-extras
No tooltips                         Tooltip wrappers on toolbar actions

The custom TreeNode.svelte component was replaced with the upstream tree-view package, which provides proper ARIA roles, keyboard navigation, and tree-line nesting out of the box. Several follow-up fixes resolved folder icons, chevrons, and a malformed closing tag.

Also fixed a content persistence bug: dirty content now saves on component destroy, not just blur—preventing data loss when switching files via the tree view.

Changelog

• Fix content editor writing to wrong Y.Doc store, making edits invisible to the filesystem API
• Fix content loss when switching files without blurring the editor first
• Replace raw HTML inputs and buttons with shared UI components
• Replace custom tree view with accessible upstream implementation
• Add tooltip hints to toolbar actions