email: charset decoding, inline attachments, UI fixes

- respect charset from Content-Type when decoding body and headers
- detect inline attachments with filenames (not just disposition: attachment)
- fix broken decodeQuotedPrintable reference in parseAttachments
- white background for email iframe (emails designed for light bg)
- sticky header in email list, scrollable message area
- arrow key navigation between emails with scroll-into-view
- /email/:id routing for deep linking to specific emails
- empty folder shows "No emails in this folder" instead of losing header
- fix attachment viewer header overlap with dialog close button

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/apps/officer-web/App.tsx

@@ -60,6 +60,7 @@ export function App() {
             <Route path="/projects/new" element={<Dashboard.NewProjectRedirect />} />
             <Route path="/projects/:id" element={<Dashboard.ProjectScreen />} />
             <Route path="/email" element={<Dashboard.EmailScreen />} />
+            <Route path="/email/:emailId" element={<Dashboard.EmailScreen />} />
             <Route path="/browser" element={<Dashboard.BrowserScreen />} />
             <Route path="/terminal" element={<Dashboard.TerminalScreen />} />
             <Route path="/auth/signout" element={<Authentication.SignoutScreen />} />