fix: open the composer for mailto links sent with SENDTO

Most apps start an email with `ACTION_SENDTO` and a `mailto:` URI. The
manifest offered the app for it, but React Native's Linking only
forwards VIEW intents, so the app opened on the inbox instead of the
composer.

MainActivity now rewrites a `SENDTO mailto:` intent into the equivalent
`VIEW mailto:` link before React Native sees it, on a cold start and
when the running app receives it. The EXTRA_EMAIL, EXTRA_CC, EXTRA_BCC,
EXTRA_SUBJECT and EXTRA_TEXT extras are folded into the link's query.
Bcc recipients of a mailto link now reach the composer too; they were
dropped for VIEW links as well.

src/navigation/__tests__/linking.test.ts

@@ -30,12 +30,32 @@ describe('parseDeepLink', () => {
       kind: 'compose',
       to: [{ email: 'a@b.co' }],
       cc: [{ email: 'c@d.co' }],
+      bcc: [],
       subject: 'Hi',
       body: undefined,
     });
     expect(parseDeepLink('mailto:nope')).toBeNull();
   });
 
+  it('reads the link MainActivity builds from a SENDTO mailto: intent and its extras', () => {
+    // SENDTO mailto: + EXTRA_EMAIL/CC/BCC/SUBJECT/TEXT, each value Uri.encode()d.
+    expect(parseDeepLink(
+      'mailto:?to=bob%40partner.example&cc=carol%40partner.example&bcc=dave%40partner.example'
+      + '&subject=Hi%20there&body=Line%201%0ALine%202',
+    )).toEqual({
+      kind: 'compose',
+      to: [{ email: 'bob@partner.example' }],
+      cc: [{ email: 'carol@partner.example' }],
+      bcc: [{ email: 'dave@partner.example' }],
+      subject: 'Hi there',
+      body: 'Line 1\nLine 2',
+    });
+    // An address in the URI itself plus EXTRA_EMAIL.
+    expect(parseDeepLink('mailto:bob@partner.example?to=eve%40partner.example')).toMatchObject({
+      to: [{ email: 'bob@partner.example' }, { email: 'eve@partner.example' }],
+    });
+  });
+
   it('rejects unknown links', () => {
     expect(parseDeepLink('bulwarkmobile://whatever')).toBeNull();
     expect(parseDeepLink('garbage')).toBeNull();
@@ -94,6 +114,17 @@ describe('handleDeepLink', () => {
     });
   });
 
+  it('passes Bcc recipients through to the composer', async () => {
+    const navigation = nav();
+    await handleDeepLink(
+      { kind: 'compose', to: [{ email: 'a@b.co' }], cc: [], bcc: [{ email: 'c@d.co' }] },
+      { navigation: navigation as never, resolveThreadId: async () => null },
+    );
+    expect(navigation.navigate).toHaveBeenCalledWith('Compose', expect.objectContaining({
+      prefillBcc: [{ email: 'c@d.co' }],
+    }));
+  });
+
   it('refuses when the linked account is not signed in', async () => {
     const navigation = nav();
     const ok = await handleDeepLink(

src/navigation/linking.ts

@@ -19,7 +19,7 @@ export type DeepLink =
   | { kind: 'contacts' }
   | { kind: 'files' }
   | { kind: 'settings'; tab?: string }
-  | { kind: 'compose'; to: EmailAddress[]; cc: EmailAddress[]; subject?: string; body?: string };
+  | { kind: 'compose'; to: EmailAddress[]; cc: EmailAddress[]; bcc?: EmailAddress[]; subject?: string; body?: string };
 
 function decodeSegment(value: string): string {
   try {
@@ -65,6 +65,7 @@ export function parseDeepLink(url: string): DeepLink | null {
       kind: 'compose',
       to: toAddresses(parsed.to),
       cc: toAddresses(parsed.cc),
+      bcc: toAddresses(parsed.bcc),
       subject: parsed.subject,
       body: parsed.body,
     };
@@ -168,6 +169,7 @@ export async function handleDeepLink(link: DeepLink, nav: DeepLinkNavigator): Pr
       navigation.navigate('Compose', {
         prefillTo: link.to,
         prefillCc: link.cc.length > 0 ? link.cc : undefined,
+        prefillBcc: link.bcc?.length ? link.bcc : undefined,
         prefillSubject: link.subject,
         prefillBody: link.body,
       });