refactor(plugin): unify the client UI entrypoint (#7474)

* fix(files): report document MIME types for binary files

File reads now carry the document's MIME type for binary files, so a file panel can
hand the bytes to a viewer that needs the real type (application/pdf).

* feat(plugins): file panels

Plugins contribute file panels with addFilePanel. A file tab hosts the panels that claim the
file: the built-in Markdown, HTML and image panels, plugin panels, and Source, Paseo's editor,
which claims every text file and is always the last candidate. The panel is the tab body;
the bar shows the document's status and a switcher between the file's panels (side by side
up to three, two on a phone, a menu beyond that).

Paseo asks which panel opens a file only when a plugin panel is among several candidates,
once per open, and remembers the answer per extension or exact filename when told to. Open
with in the tab menu and the switcher change the tab's panel; Always use remembers it.
Opening a file at a line switches the tab to Source.

Panels read and write the host's document, so dirty state, autosave and conflicts stay
Paseo's. DocumentFrame shows bytes documents such as PDFs in the browser's viewer.

Replaces the file tab's Preview/Source mode toggle.

* feat(file-panels): choose and forget file panels in Settings > Files

Settings > Files lists each extension or filename where a plugin panel competes with
another candidate, or that has a remembered panel. Each row chooses the panel that opens
it, or Ask every time; Ask again forgets the choice. Source appears as a candidate for
text files. Panels no connected host offers can be forgotten but not chosen.

* docs(plugins): file panel examples and author reference

Adds the pdf-panel and markdown-panel examples, the File panels section of the plugin
reference, and an e2e spec that installs both examples, opens a PDF in the browser
viewer, and edits Markdown through Paseo's document across the switcher, Source,
autosave and conflicts.

* feat(file-panels): panels read and write through a file handle

The file tab stats the file and mounts its panel without reading it. Every
panel, built-in or plugin, gets a PluginFile handle: readText, readBytes, url,
version-checked writeText/writeBytes (PluginFileConflictError unless force),
and watch. useFileText, useFileBytes and useFileUrl read through it, and
Loading joins @getpaseo/plugin/client/ui. Panels own their loading state.

The host document pipeline, the explorer preview cache and the preview
lifecycle are deleted. The daemon classifies a file version from a bounded
sniff and writes base64 content as bytes behind fileByteWrites.

A deleted file keeps its panel mounted; a read confirms the deletion first,
so a remove-and-rewrite is not reported as one. Native Source decodes with
builtins and highlights in idle-time slices, the lines around the opening
line first, so a large file no longer blocks the JS thread.

* fix(file-panels): Settings offers Source where the picker does, and review follow-ups

- Settings > Files offers Source on every key except those a built-in panel of
  another kind (Image) claims, so a plugin that alone claims a text extension
  shows the same choices as the picker.
- Delete the unused isSourceFilePanel.
- The Markdown example warns with the file name when its unmount write fails,
  rather than dropping the edit silently.
- Document that PluginFile stat fields are read at render time and that watch
  or the read hooks are how a panel learns they changed.

* refactor(plugin): consolidate the client UI entrypoint

docs/plugins.md

@@ -240,7 +240,7 @@ Do not put any other code modules in the plugin root.
 
 Shared files import contract helpers and types from `@getpaseo/plugin`. Server handler files import
 `PluginHandlerContext` from `@getpaseo/plugin/server`. Client files import Paseo UI from
-`@getpaseo/plugin/client/react-native`. Its `Icon` draws a Lucide name from the client's installed
+`@getpaseo/plugin/client/ui`. Its `Icon` draws a Lucide name from the client's installed
 icon set, or any `PluginIcon` through its `icon` prop; an unknown name or unsafe SVG renders nothing
 so it cannot break the plugin surface.
 Its controlled modal keeps presentation metadata on `<Modal title="…" icon={…}>` and body UI in
@@ -257,14 +257,14 @@ Classify every SDK export before adding it. All client entry points and implemen
 Zod schemas, and functions that run in both runtimes. A type-only import is still an architectural
 dependency; shared types must not refer to React components, hooks, Node APIs, or server contexts.
 
-| Entry                                                | Owns                                                                       | May depend on          |
-| ---------------------------------------------------- | -------------------------------------------------------------------------- | ---------------------- |
-| `@getpaseo/plugin`                                   | Shared data, schemas, RPC/settings definitions, runtime-neutral helpers    | Shared code only       |
-| `@getpaseo/plugin/server`                            | Server contexts, lifecycle contracts, and CLI process launch               | Shared and server code |
-| `@getpaseo/plugin/server/provider`, `/server/acp`    | Server provider contracts and adapters                                     | Shared and server code |
-| `@getpaseo/plugin/client`                            | Client contribution contexts, hooks, navigation, and UI contribution types | Shared and client code |
-| `@getpaseo/plugin/client/react-native`, `/client/ui` | Host-provided UI components                                                | Shared and client code |
-| `@getpaseo/plugin/client/host`                       | App-owned rendering integration; not a plugin-author entry                 | Shared and client code |
+| Entry                                             | Owns                                                                       | May depend on          |
+| ------------------------------------------------- | -------------------------------------------------------------------------- | ---------------------- |
+| `@getpaseo/plugin`                                | Shared data, schemas, RPC/settings definitions, runtime-neutral helpers    | Shared code only       |
+| `@getpaseo/plugin/server`                         | Server contexts, lifecycle contracts, and CLI process launch               | Shared and server code |
+| `@getpaseo/plugin/server/provider`, `/server/acp` | Server provider contracts and adapters                                     | Shared and server code |
+| `@getpaseo/plugin/client`                         | Client contribution contexts, hooks, navigation, and UI contribution types | Shared and client code |
+| `@getpaseo/plugin/client/ui`                      | Host-provided UI components                                                | Shared and client code |
+| `@getpaseo/plugin/client/host`                    | App-owned rendering integration; not a plugin-author entry                 | Shared and client code |
 
 Server code imports shared helpers from the root and server capabilities from `/server`. Client
 code imports shared helpers from the root and client capabilities from `/client`. Neither runtime