feat: add BLCVoice branding and Linux desktop identity

[BLCCoreStudio]

Summary

First productization slice for #77, stacked on PR #68.

• Replaces the temporary bootstrap-branding source with a canonical BLCVoice app icon source.
• Generates and commits deterministic Linux PNG, Windows ICO, and macOS ICNS assets.
• Wires the full platform icon set into Tauri bundle configuration.
• Adds a Linux per-user local installer that builds the release binary, installs it under ~/.local/bin, registers a blcvoice.desktop launcher and hicolor icons, and refreshes KDE/desktop caches when available.
• Keeps the visible launcher name as BLCVoice while matching the Linux binary/window identity (blcvoice) for taskbar/dock association.

Why

Real KDE/Wayland validation showed the app was being exercised through cargo run, so KDE surfaced the terminal/dev host rather than a properly registered BLCVoice desktop identity. The repository also explicitly documented the previous icon as a temporary non-branding asset.

Validation

• Platform icon generation ran successfully in GitHub Actions and committed the generated assets.
• Keep this PR draft until normal CI/bundle validation passes and the local installer is exercised on the real KDE/Wayland machine.

Part of #77.