feat: add sprag types CLI command and improve error messages

New command:
- sprag types: generates .sprag/types.pyi from the build manifest
  with type aliases for routes, mounts, actions, stores, and modules

Build pipeline:
- Auto-generate types.pyi during sprag build and sprag dev
- Add stores to the serializable manifest
- New _serializable_stores() helper in build.py

Scaffolding:
- Add _shared template directory support with merge_existing
- _render_template_dir() now supports merge_existing flag
- _merge_gitignore() helper for shared .gitignore templates

Error messages:
- Improved error messages across browser.py (debounce, throttle,
  virtual_scroll, infinite_scroll, ref), assets.py (module, normalize),
  mount.py, page.py, socket_bridge.py, stores.py

Type declarations:
- Added docstring to App dataclass
- Added docstring to Page dataclass
- Type stub hint in browser.py module docstring

Docs:
- Added sprag types to README.md CLI section
- Added full sprag types section to CLI reference docs

Author: BleedingXiko

README.md

@@ -87,6 +87,7 @@ sprag dev [--port 8000]
 sprag build               # full app (server + client)
 sprag build static        # pre-rendered static site
 sprag pack                # minify, bytecode-compile, optimize images
+sprag types               # generate .sprag/types.pyi from the build manifest
 sprag add route|mount|content <name>
 sprag routes              # list all surfaces
 sprag inspect /path       # show compiled output for a route

docs/app/content/docs/reference/cli.md

@@ -118,6 +118,33 @@ sprag pack [--dist dist] [--zip] [--dry-run] [--verbose]
 sprag pack --dist dist --zip --skip-images
 ```
 
+## `sprag types`
+
+Generate a `.sprag/types.pyi` file from the current build manifest. The generated file
+contains type aliases for routes, mounts, actions, stores, and module aliases — useful for
+IDE autocompletion and type-safe dispatch.
+
+**Usage**
+
+```bash
+sprag types [--output PATH]
+```
+
+**Flags**
+
+- `--output` — directory where `types.pyi` will be written (default: `.sprag`)
+- `--app` — explicit app import target
+- `--project-root` — project root to load from
+
+**Example**
+
+```bash
+sprag types
+```
+
+The command reads `manifest.json` from the output directory and writes `types.pyi` alongside
+it. It is also run automatically as part of `sprag build` and `sprag dev`.
+
 ## `sprag add route`
 
 Scaffold a new route under `app/routes/`.

pyproject.toml

@@ -50,4 +50,4 @@ version = {attr = "sprag.__version__"}
 include = ["sprag", "sprag.*"]
 
 [tool.setuptools.package-data]
-sprag = ["assets/**/*", "dev/templates/**/*.tmpl"]
+sprag = ["assets/**/*", "dev/templates/**/*.tmpl", "py.typed", "**/*.pyi"]