fix(reader): render inline cid: images in HTML emails

Inline images in rich HTML emails (embedded via Content-ID and
<img src="cid:...">, e.g. USPS Informed Delivery digests) were listed
under Attachments with a download button and never rendered in the body.
Two bugs combined; both are fixed here.

1. Misclassification: ImapClient classified any part with a filename as
   an attachment, sweeping inline images (which carry a filename AND a
   Content-ID under Content-Disposition: inline) into the list. A part is
   now a downloadable attachment only when its disposition is attachment,
   or it has a filename but no Content-ID; an inline image is collected
   separately and excluded from the displayed list (AttachmentDao filters
   contentId IS NULL). The Content-ID is read via MimePart.getContentID()
   so it resolves from IMAP BODYSTRUCTURE rather than a per-part header
   fetch that Angus leaves unpopulated.

2. No rendering path: HtmlBody's WebViewClient now overrides
   shouldInterceptRequest to resolve cid:<id> to the matching part's
   bytes (backing the CSP's existing cid: allowance). Content-ID is
   threaded end-to-end through AttachmentPart, Attachment,
   AttachmentEntity, and MailRepository.inlineImages(); ReaderViewModel
   surfaces the cid->bytes map to the WebView.

Schema: adds attachments.contentId (v16 -> v17, MIGRATION_16_17).

Tests: MIME-part classification (inline+cid excluded, real/disposition/
filename-only kept), a GreenMail multipart/related round-trip, the
cid->bytes resolver, repository inlineImages(), the DAO display filter,
and the v16->v17 migration.

Closes #133

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

app/src/androidTest/kotlin/org/libremail/data/local/LibreMailDatabaseTest.kt

@@ -96,6 +96,25 @@ class LibreMailDatabaseTest {
         )
     }
 
+    @Test
+    fun observeForMessageHidesInlineImagesWhileGetForMessageKeepsThem() = runBlocking {
+        val messageDao = db.messageDao()
+        val attachmentDao = db.attachmentDao()
+        messageDao.insertNew(listOf(message("acct:1")))
+        attachmentDao.insert(
+            listOf(
+                AttachmentEntity("acct:1", 0, "logo.png", "image/png", 4, contentId = "logo1"),
+                AttachmentEntity("acct:1", 1, "invoice.pdf", "application/pdf", 10, contentId = null),
+            ),
+        )
+
+        // The displayed list excludes inline cid: images (issue #133) ...
+        val displayed = attachmentDao.observeForMessage("acct:1").first()
+        assertEquals(listOf("invoice.pdf"), displayed.map { it.filename })
+        // ... while the full read keeps them so their bytes can back a cid: request.
+        assertEquals(2, attachmentDao.getForMessage("acct:1").size)
+    }
+
     @Test
     fun searchRowsAreNotInboxAndAreCleared() = runBlocking {
         val messageDao = db.messageDao()

app/src/androidTest/kotlin/org/libremail/data/local/MigrationTest.kt

@@ -169,6 +169,37 @@ class MigrationTest {
         db.close()
     }
 
+    /** v16 -> v17 (issue #133): `attachments.contentId` appears defaulting to NULL; cached rows survive. */
+    @Test
+    fun migrate16To17_addsNullContentIdToAttachments() {
+        helper.createDatabase(TEST_DB, 16).apply {
+            // v16 dropped the account tables, so a message (no FK to accounts) plus its attachment is
+            // all that's needed to exercise the attachments table rebuild.
+            execSQL(
+                "INSERT INTO messages (id, accountId, sender, senderEmail, subject, snippet, body, isHtml, " +
+                    "timestampMillis, isRead, isStarred, folder, inInbox, bodyFetched, uid) VALUES " +
+                    "('acct:INBOX:1', 'acct', 'Ada', 'ada@example.org', 'Hi', '', '', 0, 1000, 0, 0, " +
+                    "'INBOX', 1, 1, 1)",
+            )
+            execSQL(
+                "INSERT INTO attachments (messageId, partIndex, filename, mimeType, sizeBytes) " +
+                    "VALUES ('acct:INBOX:1', 0, 'report.pdf', 'application/pdf', 2048)",
+            )
+            close()
+        }
+
+        val db = helper.runMigrationsAndValidate(TEST_DB, 17, true, MIGRATION_16_17)
+
+        db.query("SELECT filename, contentId FROM attachments WHERE messageId = 'acct:INBOX:1'").use { c ->
+            assertTrue("the pre-upgrade attachment row must survive", c.moveToFirst())
+            assertEquals("report.pdf", c.getString(0))
+            assertTrue("existing attachments read a null contentId (treated as ordinary downloads)", c.isNull(1))
+            assertFalse("only the one pre-upgrade attachment row must survive", c.moveToNext())
+        }
+        assertEquals("the cached message must be untouched by 16->17", 1, db.count("messages"))
+        db.close()
+    }
+
     /** The newest schema JSON exported to app/schemas (shipped to the test APK as assets). */
     private fun latestExportedSchemaVersion(): Int {
         val schemaFolder = checkNotNull(LibreMailDatabase::class.java.canonicalName)

app/src/androidTest/kotlin/org/libremail/ui/Fakes.kt

@@ -11,6 +11,7 @@ import org.libremail.domain.model.Attachment
 import org.libremail.domain.model.Draft
 import org.libremail.domain.model.Folder
 import org.libremail.domain.model.ImapConnectionParams
+import org.libremail.domain.model.InlineImage
 import org.libremail.domain.model.Message
 import org.libremail.domain.model.OutboxMessage
 import org.libremail.domain.model.OutgoingMessage
@@ -120,6 +121,8 @@ class FakeMailRepository(
         },
     )
 
+    override suspend fun inlineImages(messageId: String): List<InlineImage> = emptyList()
+
     override suspend fun downloadAttachment(messageId: String, partIndex: Int): Result<File> =
         Result.failure(UnsupportedOperationException("not used in UI tests"))