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/NoteDocumentTests.swift

@@ -137,6 +137,44 @@ struct NoteDocumentTests {
         #expect(Note.audioRelativePath(forJSONPath: legacyJSONPath) == "1999/12/\(id.stringValue).m4a")
     }
 
+    // MARK: - Schema v4 (project)
+
+    @Test func projectCodableRoundTrip() throws {
+        var context = CaptureContext.empty
+        context.device = "iPhone"
+        let note = Note.create(text: "Ship the release notes", context: context, project: "Contextful v0.3")
+        let data = try DocumentCoder.encode(note)
+        let decoded = try DocumentCoder.decode(Note.self, from: data)
+        #expect(decoded.payload == note.payload)
+        #expect(decoded.payload.project == "Contextful v0.3")
+    }
+
+    /// A v3 file predates the `project` key. It must decode unchanged — as an
+    /// unfiled note with `project == nil` — and pass the forward gate (3 ≤ current).
+    @Test func v3DocumentDecodesWithNoProject() throws {
+        let id = ULID(millisecondsSinceEpoch: 1_700_000_000_000, randomHi: 7, randomLo: 8)
+        let json = """
+        {
+          "meta": {
+            "id": "\(id.stringValue)",
+            "schemaVersion": 3,
+            "createdAt": "2023-11-14T22:13:20Z",
+            "modifiedAt": "2023-11-14T22:13:20Z"
+          },
+          "payload": {
+            "text": "A v3 note",
+            "source": "typed",
+            "tags": [],
+            "context": { "device": "iPhone", "timeZoneID": "UTC" }
+          }
+        }
+        """
+        let note = try DocumentCoder.decode(Note.self, from: Data(json.utf8))
+        #expect(note.meta.schemaVersion == 3)
+        #expect(note.payload.project == nil)
+        #expect(note.isReadable)
+    }
+
     @Test func createIngestedDerivesCreatedAtFromULID() {
         let id = ULID(millisecondsSinceEpoch: 1_700_000_000_000, randomHi: 5, randomLo: 6)
         var context = CaptureContext.empty