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/Note.swift

@@ -45,8 +45,10 @@ struct Note: SyncDocument, VersionedDocument, Equatable {
     /// own writing, and every transcription attempt is preserved. v1/v2 files
     /// decode unchanged (missing keys ⇒ nil/empty); `migrateIfNeeded` back-fills
     /// v2 voice notes by moving a completed transcript from `text` into
-    /// `attempts` at read time.
-    static let currentSchemaVersion = 3
+    /// `attempts` at read time. v4 added optional `payload.project` — purely
+    /// additive, so v1–v3 files decode unchanged (missing key ⇒ nil) with no
+    /// read-time transformation needed.
+    static let currentSchemaVersion = 4
 
     struct Meta: Codable, Sendable, Equatable {
         var id: ULID
@@ -149,6 +151,10 @@ struct Note: SyncDocument, VersionedDocument, Equatable {
         /// Voice-note sidecar metadata, nil for a plain typed note. Optional so
         /// a v1 document (no `audio` key) decodes as a text note unchanged.
         var audio: AudioInfo?
+        /// The single project (or "category" — user's choice, see
+        /// `ProjectsNaming`) this note is filed under. Nil means unfiled.
+        /// Optional so a pre-v4 document (no `project` key) decodes unchanged.
+        var project: String? = nil
     }
 
     var meta: Meta
@@ -186,12 +192,13 @@ struct Note: SyncDocument, VersionedDocument, Equatable {
         source: Source = .typed,
         tags: [String] = [],
         context: CaptureContext,
-        audio: AudioInfo? = nil
+        audio: AudioInfo? = nil,
+        project: String? = nil
     ) -> Note {
         let now = Date()
         return Note(
             meta: Meta(id: ULID(), schemaVersion: currentSchemaVersion, createdAt: now, modifiedAt: now),
-            payload: Payload(text: text, source: source, tags: tags, context: context, audio: audio)
+            payload: Payload(text: text, source: source, tags: tags, context: context, audio: audio, project: project)
         )
     }