feat(email): add provider picker to Settings > Account

b5 of the multi-provider email roadmap (frontend). Adds EmailProviderConnections
-- one card per provider (Gmail, Outlook/Microsoft 365, generic IMAP) showing
connect status and connect/disconnect actions, mounted in SettingsView's
Account tab alongside the existing app-login GoogleAuthCard (a separate
concern: that card is sign-in identity, this is mailbox linking).

Gmail and Microsoft reuse the OAuth-popup + postMessage handshake already
built server-side (mirrors Correspondence.tsx's existing Gmail-connect flow).
IMAP has no OAuth step, so it's a plain host/port/ssl/username/password form
posting to /api/imap/connect, which verifies the connection server-side
before storing it.

Deliberately NOT touched: the Gmail-specific job-candidate-matching/review UI
in Correspondence.tsx and GmailReviewPage.tsx. That backend pipeline
(ListJobCandidateMessagesAsync, GmailReviewDecisions) is still Gmail-only by
design -- generalising it now would mean building fake UI for capabilities
Microsoft/IMAP don't have yet. This is scoped to the piece that's actually
provider-neutral: connect/disconnect status.

Verified live (backend + frontend dev servers): logged in, confirmed all
three /status calls return 200, Gmail connect-url fetch succeeds, IMAP form
submit hits /api/imap/connect and surfaces the expected 400 on a bad host.

Frontend suite: 25 suites / 57 tests green (2 new).

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

job-tracker-ui/src/components/SettingsView.tsx

@@ -22,6 +22,7 @@ import { useNavigate } from "react-router-dom";
 import { JobTableColumns } from "./JobTable";
 import ImportExportJobs from "./ImportExportJobs";
 import GoogleAuthCard from "./GoogleAuthCard";
+import EmailProviderConnections from "./EmailProviderConnections";
 import RulesSettingsCard from "./RulesSettingsCard";
 import BackupCard from "./BackupCard";
 import QuickCaptureCard from "./QuickCaptureCard";
@@ -338,6 +339,9 @@ export default function SettingsView({
       <TabPanel value={tab} index={3}>
         <AuthStatusCard />
         <GoogleAuthCard />
+        <Box sx={{ mt: 2 }}>
+          <EmailProviderConnections />
+        </Box>
       </TabPanel>
 
       <TabPanel value={tab} index={4}>