feat(accountsetup): add Yahoo app-password/2FA help in onboarding

Yahoo's guided app-password setup pointed appPasswordHelpUrl at the
generic account-security sign-in page, which assumes the user already
knows to hunt for "Create app password" once there. Point it instead at
Yahoo's own step-by-step "Generate and manage 3rd-party app passwords"
article, mirroring what #153 did for iCloud.

Yahoo does NOT gate app-password creation behind two-step verification
(verified against Yahoo's live help docs, which never list it as a
prerequisite), so — unlike Gmail and iCloud — it keeps twoFactorHelpUrl
null and shows no 2FA button. AppPasswordSetupScreen already renders the
help buttons generically from these provider fields, so no screen change
is needed; the existing PR #152 ordering (2FA link before the
app-password link) is preserved for the providers that have both.

Add a MailProviderTest assertion pinning Yahoo's new app-password URL
(mirroring the iCloud test) and extend the onboarding
yahooSetup_hasNoTwoFactorHelpLink coverage note for #155.

Closes #155

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/src/androidTest/kotlin/org/libremail/ui/onboarding/OnboardingFlowTest.kt

@@ -254,10 +254,11 @@ class OnboardingFlowTest {
         waitForText("Yahoo Mail")
         composeTestRule.onNodeWithText("Yahoo Mail").performClick()
 
-        // Yahoo's setup screen keeps its app-password link…
+        // Yahoo's setup screen keeps its app-password link (now pointing at Yahoo's step-by-step
+        // instructions article instead of the generic account-security page, issue #155)…
         waitForText(string(R.string.app_password_open_page, "Yahoo Mail"))
         // …but gains no two-factor help link: unlike Gmail and iCloud, Yahoo gates nothing on it
-        // (issue #98, #153).
+        // (issue #98, #153, #155).
         composeTestRule.onNodeWithText(string(R.string.app_password_2fa_help)).assertDoesNotExist()
         composeTestRule.onNodeWithText(string(R.string.app_password_2fa_help_icloud)).assertDoesNotExist()
     }

app/src/main/kotlin/org/libremail/domain/model/MailProvider.kt

@@ -64,7 +64,12 @@ enum class MailProvider(
     YAHOO(
         key = "yahoo",
         displayName = "Yahoo Mail",
-        appPasswordHelpUrl = "https://login.yahoo.com/account/security",
+        // Yahoo's own step-by-step "Generate and manage 3rd-party app passwords" article, not
+        // just the generic account-security sign-in page — that page assumes the user already
+        // knows to look for "Create app password" once there (issue #155). Unlike Gmail/iCloud,
+        // this article never lists two-step verification as a prerequisite for generating an app
+        // password, so there is no twoFactorHelpUrl below.
+        appPasswordHelpUrl = "https://my.help.yahoo.com/kb/mail/generate-app-specific-password-sln15241.html",
         imapHost = "imap.mail.yahoo.com",
         smtpHost = "smtp.mail.yahoo.com",
         // Yahoo documents smtp.mail.yahoo.com:465 with implicit SSL/TLS as its outgoing server.

app/src/test/kotlin/org/libremail/domain/model/MailProviderTest.kt

@@ -96,7 +96,9 @@ class MailProviderTest {
         // generic Apple ID sign-in page (issue #153).
         assertEquals("https://support.apple.com/en-us/102660", MailProvider.ICLOUD.twoFactorHelpUrl)
 
-        // Yahoo gates nothing on two-factor, so it must not grow the extra link.
+        // Yahoo gates nothing on two-factor — reconfirmed directly against Yahoo's own help docs,
+        // which never list two-step verification as a prerequisite for generating an app password
+        // (issue #155) — so it must not grow the extra link.
         assertNull(MailProvider.YAHOO.twoFactorHelpUrl)
     }
 
@@ -105,6 +107,14 @@ class MailProviderTest {
         assertEquals("https://support.apple.com/en-us/102654", MailProvider.ICLOUD.appPasswordHelpUrl)
     }
 
+    @Test
+    fun `yahoo app-password help points at Yahoo's step-by-step instructions, not the generic sign-in page`() {
+        assertEquals(
+            "https://my.help.yahoo.com/kb/mail/generate-app-specific-password-sln15241.html",
+            MailProvider.YAHOO.appPasswordHelpUrl,
+        )
+    }
+
     @Test
     fun `createAccount trims the email and derives a stable id and display name`() {
         val account = MailProvider.GMAIL.createAccount("  User@Gmail.com  ")