feat: add auto-join link sharing and companion auto-connect

[bjowe]

• Add /auto-join route that joins calls from URL params
• Add share icon next to "Calls" title opening an auto-join link modal
• Auto-join link modal optionally appends a companion WebSocket URL
• Add useAutoJoinRestore hook to restore calls from localStorage on reload
• Add companionAutoConnectUrl to ConnectToWSButton for auto-connecting WS
• Add titleAction prop to PageHeader for injecting content next to title

[bjowe]

What
Adds an auto-join feature that lets users share a single URL that automatically joins a specific set of production lines, with optional username and Companion WebSocket connection included in the link.

Changes

• Add /auto-join?calls=:,... route that joins one or more calls on arrival and navigates to the calls page
• Add a share icon next to the "Calls" title that opens a modal for generating the auto-join link
• Auto-join link modal includes:
• Optional username field — appended as ?username= so the recipient joins with that name instead of their saved settings
• Optional companion connection checkbox — appends &companion=ws://127.0.0.1:12345 for automatic Companion WebSocket connection
• Add useAutoJoinRestore hook that restores saved calls from localStorage when the user navigates back to the calls page after leaving
• Add companionAutoConnectUrl support in ConnectToWSButton — reads a WebSocket URL from localStorage and auto-connects to Companion on mount
• Add titleAction prop to PageHeader to allow injecting content next to the title
• Fix crash in api.ts when VITE_BACKEND_URL is not set (.replace() called on undefined)
• Fix missing trailing slash in API_URL when VITE_BACKEND_API_VERSION is set without one

How to test

1. Join one or more production lines
2. Click the share icon next to the Calls heading
3. Optionally enter a username and/or check Include companion connection
4. Copy and open the generated link in a new tab — the calls should join automatically