Fix MV3 bugs, add preferences page, shortcuts, and duplicate-tab closing

[mos3abof]

Summary

• Split manifest.json into manifest.firefox.json and manifest.chrome.json; Chrome now correctly uses service_worker and action (MV3); Firefox gains a gecko ID for AMO publishing
• Fixed contextMenus.onclick (broken in MV3) → onClicked listener registered in onInstalled, as required by Chrome service workers
• Guarded against undefined tab URLs (new tab pages, internal browser pages)
• Replaced deprecated event.keyCode with event.key in popup
• Batch-remove tabs with a single chrome.tabs.remove([...ids]) call instead of one per tab
• Context menu now surfaces on image, link, and frame contexts, not just page
• New "Close duplicate tabs" context menu item
• New options page: exclude current tab, exclude pinned tabs, exclude extension pages, case-sensitive matching (all backed by storage.sync)
• Keyboard shortcut Ctrl+Shift+X / ⌘+Shift+X to open the popup without clicking the toolbar icon
• Esc now closes the popup as the footer text has always promised
• Updated stale homepage_url and in-popup website link to the new GitHub location
• Makefile: separate build-firefox / build-chrome targets, versioned zip filenames (power-close-1.1.0-*.zip), and lint / run targets via web-ext
• Bumped version to 1.1.0 across both manifests

Test Plan

• make build produces dist/power-close-1.1.0-firefox.zip and dist/power-close-1.1.0-chrome.zip
• Load Firefox build via about:debugging and verify keyword/domain closing works
• Load Chrome build via chrome://extensions and verify keyword/domain closing works
• Ctrl+Shift+X opens the popup; Esc closes it
• Right-clicking an image or link shows both context menu items
• "Close duplicate tabs" leaves one copy of each URL open
• Options page persists settings across browser restarts
• Pinned tabs survive when "Don't close pinned tabs" is enabled
• Current tab survives when "Don't close the current tab" is enabled
• make lint passes web-ext lint without errors