Add Flash Card plugin with HTML generation and key point extraction

- Introduced a new Flash Card plugin that generates visually appealing flashcards from text input.
- Implemented functionality to extract key points and categories for efficient learning.
- Added a new Python file for the plugin logic and a corresponding image asset.
- Removed outdated README files for the previous knowledge card plugin.

plugins/actions/flash-card/README.md

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+# Flash Card
+
+Generate polished learning flashcards from any text—title, summary, key points, tags, and category—ready for review and sharing.
+
+![Flash Card Example](flash_card.png)
+
+## Highlights
+
+- **One-click generation**: Drop in text, get a structured card.
+- **Concise extraction**: 3–5 key points and 2–4 tags automatically surfaced.
+- **Multi-language**: Choose target language (default English).
+- **Progressive merge**: Multiple runs append cards into the same HTML container; enable clearing to reset.
+- **Status updates**: Live notifications for generating/done/error.
+
+## Parameters
+
+| Param               | Description                                                  | Default |
+| ------------------- | ------------------------------------------------------------ | ------- |
+| MODEL_ID            | Model to use; empty falls back to current session model      | empty   |
+| MIN_TEXT_LENGTH     | Minimum text length; below this prompts for more text        | 50      |
+| LANGUAGE            | Output language (e.g., en, zh)                               | en      |
+| SHOW_STATUS         | Whether to show status updates                               | true    |
+| CLEAR_PREVIOUS_HTML | Whether to clear previous card HTML (otherwise append/merge) | false   |
+| MESSAGE_COUNT       | Use the latest N messages to build the card                  | 1       |
+
+## How to Use
+
+1. Install and enable “Flash Card”.
+2. Send the text to the chat (multi-turn supported; governed by MESSAGE_COUNT).
+3. Watch status updates; the card HTML is embedded into the latest message.
+4. To regenerate from scratch, toggle CLEAR_PREVIOUS_HTML or resend text.
+
+## Output Format
+
+- JSON fields: `title`, `summary`, `key_points` (3–5), `tags` (2–4), `category`.
+- UI: gradient-styled card with tags, key-point list; supports stacking multiple cards.
+
+## Tips
+
+- Very short text triggers a prompt to add more; consider summarizing first.
+- Long text is accepted; for deep analysis, pre-condense with other tools before card creation.