Projects: classes as entities, and the sidebar fits short windows

A project is a class with facts (description, status, kind, party,
dates, links, notes) in one project.set event behind the format
feature projects. Four tools (books.add_project, update_project,
projects, project); documents and decisions link to projects; B-37:
on-spec costs are expensed as incurred, repayment is revenue, and the
unrecovered cost is never below zero (Kani). Viewer: a Projects list
and a project page with its figures, by month and by account. The
sidebar measures its footer instead of reserving a fixed 172 px.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01T3gXMVt7XJPi6ens1or9Dj

crates/ledger-core/src/error.rs

@@ -654,14 +654,23 @@ pub enum LedgerError {
     #[error("unknown payroll run id {0}")]
     UnknownPayrollRun(u64),
 
-    /// Class names must not be empty.
-    #[error("class name must not be empty")]
+    /// Class (project) names must not be blank.
+    #[error("a project (class) name is blank")]
     EmptyClassName,
 
     /// The referenced class does not exist.
     #[error("unknown class id {0}")]
     UnknownClass(u32),
 
+    /// Another class (project) already has this name, ignoring capitals
+    /// (rules.md B-37).
+    #[error("a project named {0:?} already exists: names are unique, ignoring capitals")]
+    DuplicateClassName(String),
+
+    /// A project's facts the books can't take (rules.md B-37): the reason.
+    #[error("{0}")]
+    InvalidProject(String),
+
     /// A setting change the books can't honor (crate::settings): an
     /// unknown key, a value its key refuses, or settings that wouldn't
     /// hold together.