Add Projects: third tab, swipe-to-file, per-project note capture

Notes gain an optional project (schema v4, additive). Projects derive
from note values like tags — no separate entity. Tab naming
(Projects/Categories) is a Settings choice applied across the UI.

Claude-Session: https://claude.ai/code/session_014esDWi42URLEC6Cj17hGQ3

Notes/Model/NoteEntity.swift

@@ -10,6 +10,8 @@ final class NoteEntity {
     var text: String = ""
     var sourceRaw: String = Note.Source.typed.rawValue
     var tags: [String] = []
+    /// The project (or "category") this note is filed under, nil when unfiled.
+    var project: String?
     var createdAt: Date = Date()
     var modifiedAt: Date = Date()