docs: add screenshots, move encryption to docs/cipher.md, and update readme

docs/cipher.md

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+# Encryption
+
+### PLAIN
+No encryption. Encodes text directly.
+
+### SPECK32/64 ECB
+Block cipher in ECB mode. Each character is encrypted independently — identical plaintext characters produce identical ciphertext. Useful when you want varied-looking output from the same input, but **not secure under real threat models** since patterns in the plaintext leak through.
+
+### SPECK48/96 CTR
+Stream cipher mode with a random 24-bit nonce prepended to the output. Identical plaintexts produce different ciphertexts each time. Significantly more secure than ECB. **Rotate your password after roughly every 100 encryptions** to avoid nonce reuse risk within the 24-bit space.