Register a voltius:// URL scheme (deep-link infrastructure)

[kipavy]

Part of the team-ux initiative (brainstorm 2026-07-24).

Referenced by: #65.

Problem

No custom URL scheme is registered anywhere (src-tauri/ only handles http/https; "deep-link" appears solely as an internal-navigation comment in uiStore.ts). That blocks any "click/scan → open the app straight into the right screen" flow — starting with joining a live session (#65), but reusable for vault invites, hosts, etc.

Proposed scope

• Register the voltius:// scheme across desktop (macOS / Windows / Linux) and mobile via Tauri's deep-link plugin.
• A router that maps inbound URLs to in-app navigation (e.g. voltius://join/<code> → resolve code → open the session; extensible to voltius://vault/<id>, voltius://host/<id>).
• Cold-start (app launched by the link) and warm (already running) handling; queue the intent until the app is signed in / ready.

Decisions (brainstorm 2026-07-24)

• Split out of Multiplayer session share: human-friendly code + auto-copy + QR (drop the raw id:token string) #65 because it's reusable infrastructure, not session-specific, and carries real cross-platform registration work that shouldn't gate the code+QR core.

Still open

• Tauri deep-link plugin wiring per platform (incl. mobile intent filters / URL types).
• URL grammar + a single inbound router.
• Security: never auto-act on a deep link without the normal auth/consent gates.
• Universal-link (https://) upgrade later, for link previews / no scheme prompt.