Generate bundle icons + drop unsigned updater for v1.0.0 #4795

Originally written by @ccantynz-alt on GitHub.
Imported from https://github.com/ccantynz-alt/voxlen/pull/6

Second round of CI fixes. The first PR fixed TypeScript + the iOS runner; this one fixes the Tauri bundle step itself.

Root causes

• src-tauri/icons/ directory didn't exist but tauri.conf.json referenced 5 bundle icons + 1 tray icon. Tauri's bundler fails loudly when it can't find them.
• tauri-plugin-updater was wired in but had pubkey: "REPLACE_WITH_TAURI_UPDATER_PUBKEY" — that placeholder fails tauri::generate_context!() at compile time.

Fixes

• Generated the full icon set via npx tauri icon from a 1024×1024 Marco Reid branded source (indigo rounded square, white mic glyph). Desktop .icns/.ico/PNGs + mobile variants.
• Removed tauri-plugin-updater from Cargo.toml, lib.rs, capabilities/default.json, and tauri.conf.json. Can re-add once we have a real Ed25519 keypair and a live releases.marcoreid.com endpoint.
• Regenerated Cargo.lock.

After this lands on main, re-run the workflow from https://github.com/ccantynz-alt/voxlen/actions.