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

NotesTests/NoteMapperTests.swift

@@ -43,6 +43,21 @@ struct NoteMapperTests {
         #expect(all.first?.tags == ["coffee", "people"])
     }
 
+    @Test func upsertWritesProjectAndNoteRestoresIt() throws {
+        let (container, context) = try makeStore()
+        defer { withExtendedLifetime(container) {} }
+        var note = makeNote()
+        note.payload.project = "Kitchen Remodel"
+        NoteMapper.upsert(note, relativePath: note.relativePath, into: context)
+
+        let entity = try #require(NoteMapper.fetch(id: note.meta.id.stringValue, in: context))
+        #expect(entity.project == "Kitchen Remodel")
+
+        let restored = try #require(NoteMapper.note(from: entity))
+        #expect(restored.payload.project == "Kitchen Remodel")
+        #expect(restored.payload == note.payload)
+    }
+
     @Test func entityRoundTripsBackToNote() throws {
         let (container, context) = try makeStore()
         defer { withExtendedLifetime(container) {} }