fix(release): mainBinaryName so Flatpak packaging finds the binary

[johnjazzi]

Why

The release run for #85 failed in build-flatpak (both arches):

ar -x canonic.deb
install: cannot stat 'usr/bin/canonic': No such file or directory
Error: module canonic: Child process exited with code 1

The Tauri .deb named the executable after the Cargo bin (app), but the
Flatpak manifest, command:, .desktop and icon paths all reference
canonic. With mainBinaryName unset, Tauri fell back to the crate bin name.

Fix

Set "mainBinaryName": "canonic" in tauri.conf.json. The bundled binary,
.desktop and icons are now canonic.*, satisfying every manifest reference.
Updater asset names are productName-derived (canonic.app.tar.gz,
canonic_*_x64-setup.exe, *.AppImage), so they're unchanged and latest.json
URLs still resolve.

Verified: built .deb from the failed run contains usr/bin/app; manifest
expects usr/bin/canonic. cargo check passes; 279 tests pass.

Context

Completes the updater/release fixes whose siblings already merged via #85:

• clippy -D warnings errors (FETCH_HEAD format!, redundant u64 cast)
• core:window:allow-set-focus capability (titlebar setFocus)
• verify-manifest post-release smoke test for latest.json

Merging this triggers a clean release that should publish a signed latest.json.

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