feat: add styles.entries config for injecting global stylesheets

[stevenle]

Motivation

• Provide a way for projects to manually declare global stylesheet files to be injected as <link rel="stylesheet"> tags on every rendered page.
• Ensure manually declared styles can be ordered before auto-collected CSS dependencies and are normalized relative to the site's base path.

Description

• Added a new styles section to RootUserConfig with entries?: string[] and documentation on normalization and de-duplication.
• Updated the Renderer to merge configured stylesheet entries into the cssDeps set before collecting auto-collected CSS, so manual entries appear first and duplicates are de-duped.
• Implemented getConfiguredStyleEntries() to normalize configured entries against rootConfig.base and introduced normalizeStyleEntry(entry, basePath) to produce URL-safe paths.
• Left existing CSS collection and element/script dependency handling intact while ensuring manual styles are included in the generated styleTags.

Testing

• No automated tests were added or executed as part of this change.

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Codex Task

[stevenle]

@jeremydw here's the codex recommended pr for the #925. once this is in i think i'll need to update #928 (root-tailwind plugin) to use this config instead of it creating a vite plugin for auto tailwind css imports.

[jeremydw]

Hm OK - I think it's alright, some feedback:

1. Better name than entries? Not sure.
2. What if the file should be imported in some routes but not all...? Is there a pattern we can follow for that or should we treat that as out of scope?

Of course I'm OK merging as is because I think getting the feature in is pretty important but just wanted to point these 2 things out

[stevenle]

i think codex chose "entries" because it's consistent with vite naming, the only other thing i could think of is "entrypoints". "include" could work but it's a little more ambiguous for its purpose (e.g. someone may think to use "include" as a way of saying "these are all the css files that should be available in the site").

out of scope for limiting per route, we should continue to use import '<path>.css' for that. this should be intended as "global css" for configuration purposes (e.g. setting up design system tokens, css cascade layers, tailwind themes, etc.)

[jeremydw]

lgtm