Share/load URL carries filename (preserves language)

[acbcdev]

What to build

Carry the tab's filename through the share link so a shared .md/.html opens with the correct language (since language is derived from the name).

• handleShare appends &name=<encoded tab.name> to the existing /?code=<base64> link.
• The URL load path (App.tsx) reads the name param and uses it as the new tab's name; its extension determines the language.
• Backward compatible: links without a name param fall back to shared.ts (current behavior).

Acceptance criteria

• Share link includes the encoded filename
• Opening a shared link restores the tab with its original name and derived language
• Sharing a markdown tab opens it as markdown (preview) on the other end
• Old links (no name param) still load as shared.ts

Blocked by

• Derive tab language from filename extension + Plaintext fallback #22