release infra: build scripts, Firefox signing, plugin fixes

- .gitignore: release/, .env, *.xpi, node_modules/
- .env.example: template for AMO credentials
- extension-firefox/package.json: web-ext scripts (lint, sign)
- extension-firefox/manifest.json: gecko.id + update_url + data_collection_permissions
- scripts/build.sh: renamed binaries (verstak, verstak-server), release target
- scripts/sign-firefox-xpi.sh: AMO signing with --self-hosted
- scripts/release-firefox-xpi.sh: signed XPI + updates.json generation
- scripts/release.sh: full release pipeline (DEB/RPM/checksums/git tag)
- VERSION: 0.1.0
- README.md: Firefox extension & release sections

Plugin fixes:
- internal/core/plugins/manager.go: auto-create .verstak/plugins/ dir
- frontend/src/lib/SettingsSidebar.svelte: remove 'plugins' from disabled + active left border
- frontend/src/lib/SettingsPlugins.svelte: force ListPlugins refresh on toggle
- frontend/src/lib/SettingsWindow.svelte: onPluginToggle callback
- frontend/src/App.svelte: refreshSystemViews on plugin toggle
- frontend/src/lib/CalendarPluginPage.svelte: visible error icon

.env.example

@@ -0,0 +1,22 @@
+# Verstak Firefox Extension — environment template
+# Copy to .env and fill in real values.
+# DO NOT commit .env — it contains AMO secrets.
+
+# AMO API credentials (from https://addons.mozilla.org/developers/)
+WEB_EXT_API_KEY=
+WEB_EXT_API_SECRET=
+
+# Build channel: "unlisted" for self-distributed builds
+WEB_EXT_CHANNEL=unlisted
+
+# Source directory for the Firefox extension
+WEB_EXT_SOURCE_DIR=extension-firefox
+
+# Output directory for signed XPI artifacts
+WEB_EXT_ARTIFACTS_DIR=web-ext-artifacts
+
+# Base URL for Firefox self-hosted updates
+VERSTAK_FIREFOX_UPDATE_BASE_URL=https://mirv.top/verstak/firefox
+
+# Optional: HTTP proxy for AMO API requests (e.g. http://localhost:12334)
+# WEB_EXT_API_PROXY=