Polish Pair black chat shell to match real features

[Dahhrk]

Summary

• Rebuild Pair's desktop UI as a true-black Obsidian DM shell (rail + sidebar + chat) with purple accents, keeping only controls that map to real Pair behavior.
• Remove unused/fake chrome (Friends tab, idle mute/share clutter, Discord-style junk) and wire mute, deafen, share, call, volume, settings, and composer actions to existing app.js handlers.
• Keep Grid/Focus/Fullscreen and screen quality controls limited to active screen share; preserve all IDs Electron and Pair wiring still depend on.

Test plan

• Open Pair (npm start or static index.html) and confirm Obsidian true-black shell with purple selected accents and no overlapping controls
• Add Friend / invite flow opens and closes correctly; connection status updates the DM user bar
• Chat composer order is + · input · GIF · emoji · Send; attach file, emoji, and GIF still work
• Join voice: voice strip appears with volume + leave; user-bar mute/deafen work; leave ends the call
• Screen share: quality controls appear only while sharing; Grid/Focus/Fullscreen only when video is active; stop share via user-bar/header
• Settings tabs (profile, voice, appearance including Obsidian, privacy, advanced, screen sharing) all function
• node --check app.js passes

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