i18n: complete English localization and add German translation

[GHJoe23]

Summary

This PR completes the English localization and adds a full German (de) translation to the Blinko Extension. It also localizes the remaining hard-coded Chinese strings, especially the default prompt templates used for page summarization.

Changes

• Extended _locales/en/messages.json and _locales/zh_CN/messages.json with missing notification keys and new prompt/status keys.
• Added _locales/de/messages.json with a complete German translation (216 keys).
• Localized default prompt template names and contents in js/settings.js.
• Localized default tags (#Web/Summary, #Web/Excerpt, etc.) via i18n keys.
• Replaced the hard-coded Chinese fallback prompt in js/api.js with a localized key.
• Replaced hard-coded Chinese UI/status/notification strings in js/contextMenu.js, js/popup.js, js/summary.js, js/quickNote.js, js/background.js, js/settings.js, js/api.js, js/messageHandler.js, js/settings-page.js, js/floatingBall/floatingBallHandler.js, and js/content/contentExtractor.js with chrome.i18n.getMessage calls.
• Migrated %s placeholders to Chrome i18n-compatible $1/$2/$3 placeholders in notification and error messages.
• Validated all locale JSON files and verified full key parity across en, de, and zh_CN.

How to Test

1. Load the extension unpacked in Chrome from the repository root.
2. Switch Chrome language to English, German, or Simplified Chinese.
3. Open the popup and settings pages; verify all text is in the selected language.
4. Trigger a context-menu action and verify notifications/status messages are localized.
5. Check that default prompt templates are in the selected language.

Notes

• No functional behavior was changed.
• Existing user settings are preserved; only default strings for new installs or resets are affected.

Credits

• The initial code changes in this PR were created with Kimi Code 0.16.0 using the K2.7 Code High Speed model.
• The German (de) translation and the extension behavior were manually tested by a human.