refactor(orbit): switch share link to psysonic2- magic string format

Aligns orbit invites with the existing magic-string family (psysonic1- for
server invites, psysonic2- for library shares) by folding orbit into the
psysonic2- payload as a new k:'orbit' variant. The JSON body is
intentionally extendable so future layers (passwords, permissions,
invite expiry) can be added without a format migration.

- SharePayloadV1 split into EntitySharePayloadV1 + OrbitSharePayloadV1
- decodeSharePayloadFromText filters orbit out; orbit has its own decoder
- applySharePastePayload param narrowed to EntitySharePayloadV1
- buildOrbitShareLink / parseOrbitShareLink kept as thin wrappers
- slug parameter dropped (magic string is opaque, so the slug was
  cosmetic-only in the old URL form); slugifyOrbitName removed as dead code
- joinModalLinkPlaceholder updated in all 8 locales

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/components/OrbitStartModal.tsx

@@ -9,7 +9,6 @@ import {
   startOrbitSession,
   buildOrbitShareLink,
   generateSessionId,
-  slugifyOrbitName,
 } from '../utils/orbit';
 import { randomOrbitSessionName } from '../utils/orbitNames';
 import { useAuthStore } from '../store/authStore';
@@ -44,8 +43,8 @@ export default function OrbitStartModal({ onClose }: Props) {
   const onLan      = isLanUrl(serverBase);
 
   const shareLink = useMemo(
-    () => buildOrbitShareLink(serverBase, sid, slugifyOrbitName(name)),
-    [serverBase, sid, name],
+    () => buildOrbitShareLink(serverBase, sid),
+    [serverBase, sid],
   );
 
   const writeLinkToClipboard = async (): Promise<boolean> => {