Fix broken DM rendering

Currently NostrDB does not seem to handle encryption/decryption of DMs.
Since NostrDB now controls the block parsing process and fetches note
contents directly from the database, we have to add a specific condition
that injects decrypted content directly to the ndb content parser.

This is done in conjunction with some minor refactoring to `NdbBlocks`
and associated structs, as in C those are separated between the content
string and the offsets for each block, but in Swift this is more
ergonomically represented as a standalone/self-containing object.

No changelog entry is added because the previously broken version was
never released to the public, and therefore this fix produces no
user-facing changes compared to the last released version.

Changelog-None
Closes: https://github.com/damus-io/damus/issues/3106
Signed-off-by: Daniel D’Aquino <daniel@daquino.me>

damusTests/NoteContentViewTests.swift

@@ -342,6 +342,20 @@ class NoteContentViewTests: XCTestCase {
         XCTAssertTrue((parsed.blocks[0].asURL != nil), "NoteContentView does not correctly parse an image block when url in JSON content contains optional escaped slashes.")
     }
     
+    /// Quick test that exercises the direct parsing methods (i.e. not fetching blocks from nostrdb) from `NdbBlockGroup`, and its bridging code with C.
+    /// The parsing logic itself already has test coverage at the nostrdb level.
+    func testDirectBlockParsing() {
+        let kp = test_keypair_full
+        let dm: NdbNote = NIP04.create_dm("Test", to_pk: kp.pubkey, tags: [], keypair: kp.to_keypair())!
+        let blocks = try! NdbBlockGroup.from(event: dm, using: test_damus_state.ndb, and: kp.to_keypair())
+        XCTAssertEqual(blocks.blocks.count, 1)
+        
+        let post = NostrPost(content: "Test", kind: .text)
+        let event = post.to_event(keypair: kp)!
+        let blocks2 = try! NdbBlockGroup.from(event: event, using: test_damus_state.ndb, and: kp.to_keypair())
+        XCTAssertEqual(blocks2.blocks.count, 1)
+    }
+    
     func testMentionStr_Pubkey_ContainsAbbreviated() throws {
         let compatibleText = createCompatibleText(test_pubkey.npub)