fix: Context.BaseURL() discards path when HOST_MODE=single

[tejasae-afk]

When HOST_MODE=single and BASE_URL contains a path component (e.g. https://example.com/feedback), Context.BaseURL() was returning only the scheme and host (https://example.com), silently dropping the path.

This broke server-side redirects (sign-in, OAuth callbacks, delete-post flows, etc.) and the baseURL value injected into the frontend via the renderer — all of which call c.BaseURL(). Users hosted behind a reverse proxy under a sub-path would land on the root of the site instead of the configured sub-path after any redirect.

The root cause is a split between two BaseURL implementations. The package-level web.BaseURL(ctx) already handled single-host mode correctly by returning env.Config.BaseURL intact, but Context.BaseURL() always delegated to Request.BaseURL(), which only constructs scheme://host:port with no path.

The fix is one extra check at the top of Context.BaseURL():

func (c *Context) BaseURL() string {
    if env.IsSingleHostMode() {
        return env.Config.BaseURL
    }
    return c.Request.BaseURL()
}

This makes Context.BaseURL() consistent with web.BaseURL(). Multi-tenant behaviour is unchanged.

A test covering the single-host-with-path case is included.

Fixes #1452.

you know the codebase far better than I do, happy to adjust anything here.