feat(code): add Linux AppImage build target

[gantoine]

Summary

• Adds @reforged/maker-appimage and registers it in forge.config.ts so Forge produces x64 + arm64 Linux AppImages alongside the existing macOS DMG/ZIP and Windows installer.
• Adds a publish-linux matrix job (ubuntu-latest + ubuntu-24.04-arm) to code-release.yml that installs the AppImage tooling (squashfs-tools, zsync, libfuse2t64) and runs electron-forge publish. finalize-release now depends on this job too.
• Adds a pnpm --filter code make:linux script that builds the AppImage from macOS via Docker (tar-streamed source, container-side node_modules) for local verification, since electron-forge can't natively cross-build AppImages from darwin.
• Drive-by: removes a duplicate const { isOnline } = useConnectivity() in SessionView.tsx left behind by a merge collision between fix(sessions): preserve prompt when sending while offline #1971 and fix(sessions): disable message send when offline #2121 — was failing typecheck on main.

Test plan

• Push a throwaway v0.0.0-linuxtest1 tag and verify publish-linux produces both x64 and arm64 AppImages in the draft GitHub release
• Confirm finalize-release waits for all three platform jobs before un-drafting
• Locally: pnpm --filter code make:linux produces an AppImage under apps/code/out/make/ (requires Docker Desktop ≥ 8 GB memory)
• Launch the produced AppImage on a Linux box and confirm the app boots
• Verify existing macOS DMG and Windows installer release flows still publish (no changes to those job definitions, but matrix needs was extended)

Notes

• Linux artifacts are unsigned (no gpg signing wired up); flag for follow-up if signed Linux builds are needed.
• libfuse2t64 is the Ubuntu 24.04 package name; if ubuntu-latest is bumped past 24.04 in the future, that line may need adjusting.

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