feat: convert all slide panels to tabbed modals with shared TabbedModal component

Extract reusable TabbedModal component (portal, backdrop, tab bar with icons/badges/underline, scrollable body, footer) and convert all 9 detail panels from SlidePanel+CollapsibleSection to tabbed modal layout:
- PostDetailPanel (5 tabs), TaskDetailPanel (3), ProjectEditPanel (2)
- TrackDetailPanel (2), CampaignDetailPanel (3), TeamMemberPanel (3)
- TeamPanel (2), IssueDetailPanel (4), ArtefactDetailPanel (4)
Also adds post versioning system (server routes + frontend).

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

client/src/i18n/en.json

@@ -915,5 +915,22 @@
   "review.confirmApprovePostDesc": "Are you sure you want to approve this post?",
   "review.confirmRejectPostDesc": "Are you sure you want to reject this post? Please provide feedback explaining why.",
   "review.feedbackRequired": "Feedback (required)",
-  "review.feedbackRequiredError": "Please provide feedback when rejecting"
+  "review.feedbackRequiredError": "Please provide feedback when rejecting",
+  "posts.versions": "Versions",
+  "posts.newVersion": "New Version",
+  "posts.createNewVersion": "Create New Version",
+  "posts.createVersion": "Create Version",
+  "posts.creatingVersion": "Creating...",
+  "posts.whatChanged": "What changed in this version?",
+  "posts.copyLanguages": "Copy languages from previous version",
+  "posts.languages": "Languages",
+  "posts.addLanguage": "Add Language",
+  "posts.selectLanguage": "Select a language...",
+  "posts.enterContent": "Enter the content in this language...",
+  "posts.noLanguages": "No languages added yet",
+  "posts.noVersions": "No versions yet. Create one to start managing multilingual content and media.",
+  "posts.deleteLanguage": "Delete this language?",
+  "posts.deleteLanguageConfirm": "This will remove the language content from this version.",
+  "posts.media": "Media",
+  "posts.noMedia": "No media files uploaded"
 }