[FR] Deep link endpoint for external clippers

### Description

Add a appflowy-flutter://new deep link endpoint that allows external tools (browser extensions, clippers, scripts) to create documents inside AppFlowy without requiring cloud authentication or a REST API.
Supported query parameters:

appflowy-flutter://new
?workspace_id= # optional – switch to target workspace first
&parent_view_id= # optional – target space / folder; falls back to current space
&name= # optional – document title; defaults to "New Note"
&content=<url-encoded-markdown> # optional – document body (URL-encoded Markdown)
&clipboard # optional flag – read Markdown from system clipboard
# (takes precedence over &content when both are present)

### Impact

Enables a fully local, privacy-preserving clipping workflow with no data transiting through external services — a common pattern in the ecosystem. This unlocks support in browser extensions and third-party tools without requiring users to run AppFlowy Cloud.

### Additional Context

- The existing appflowy-flutter:// infrastructure already handles auth callbacks, so the URI scheme handler is already in place

- A feature request for this workflow was raised on [Clipper for AppFlowy](https://github.com/AppFlowy-IO/AppFlowy/issues#) — this PR is a direct response to it

- Alternatives considered: local REST API on a localhost port, IPC from a browser extension — both require more invasive changes; a URI scheme handler is simpler and cross-platform