Add endpoint to get the server settings (and limits)

the sample has a comment that says:

# Maximum length for custom IDs.
# Optional; default is 2500

but in the code the actual default is 500

const fn max_custom_id_length_default() -> usize {
	500
}

Currently random IDs are generated if the custom ID is not specified.
If there is a conflict when randomly generating a link, a link can't be created.
Conflicts should be extremely rare with 48 random bits for each link currently, so we should be able to just retry 2-3 times before giving up.

"max_usage" field has inconsistent width

currently the frontend needs to be build first with trunk, then the backend can be build. This could be made easier by making one workspace depend on another and also get cargo to build with trunk

affects the Dockerfile, Github Actions and Readme

The advanced mode on the website currently does not have a field for specifying a custom ID

The endpoint for retrieving the server config is missing from the api docs

Cleanup the code a bit

currently there is just an empty page, there should be a page that says that this page does not exist or redirect to the main page

If you enter a valid date in advanced mode, it gets marked as invalid.

Two cases come to mind:
* a link with a negative or 0 valid_for
* a link with max_uses of 0 or negative numbers

mozilla docs

It'd be best to first check if the .env file exists before calling dotenv::dotenv()

include attributions in about or create a separate card

Display a tooltip explaining the inputs and maybe also display the server default

also set min and max length on fields from server_config

If the link field in the json body for a custom link is an empty string or other non valid URL, the server happily accepts it and creates an (therefore invalid) shortened url

The website always sets the max_uses field to 0, no matter which number is actually entered

when sending a json with either valid_for or max_uses set to null, the return code is 400 Bad Request and a serde error message is returned in the response. The 400 may be wanted although null fields could just be treated as not existing, but the error message should not be a serde parsing error

reproduce with:
http POST https://s.flamion.dev/custom link=do_not_care max_uses=null valid_for=null
or
curl --request POST --header "Content-Type: application/json" --data '{"link":"do_not_care","valid_for":null,"max_uses":null}' https://s.flamion.dev/custom

Make the valid duration selector switchable between a date selector and a field to enter something like 3 hours or 30 minutes (maybe even just in milliseconds)

currently the api is documented via a seperate openai documentation file. With a crate like utoipa the documentation can be added directly to the endpoint definitions and easily serverd with utoipa-swagger-ui

The assets (html, js and css) should be embedded in the binary at compile time so shorty works as a standalone binary.

commit 52cecc7 changed something that broke shorty, for refernce 5358232 works as expected

Allow any CORS origin to make the API callable from other websites too (if there's a custom website on a different origin, or for example things like swagger API).
We could just utilise the actix CORS middleware

We need to add a config option and potentially also display the maximum valid duration on the website somehow

see title

example: https://a.b/stats/34kv8e2n

returns something like

{
    "visits": 15,
    <settings for link>,
    ...
}

link config would then need an added "no counting" option or similar

Check if a fields content is invalid, and if so, make the field border red

When enabling the advanced mode, the entire layout of the page shifts