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/locales/en.ts

@@ -1505,7 +1505,7 @@ export const enTranslation = {
     joinModalTitle: 'Join an Orbit session',
     joinModalSub: 'Paste the invite link your host sent you.',
     joinModalLinkLabel: 'Invite link',
-    joinModalLinkPlaceholder: 'psysonic2://orbit/…',
+    joinModalLinkPlaceholder: 'psysonic2-…',
     joinModalLinkHelper: 'Works with any valid Orbit link. Must point to the server you\'re currently signed into.',
     joinModalPasteTooltip: 'Paste from clipboard',
     joinModalSubmit: 'Join',