docs: document chat-scoped Fast mode

[biswaroop1547]

Summary

• Documents the chat composer model-mode dropdown with High and Fast choices.
• Clarifies that Fast mode is chat-specific, can change between messages, and new chats start in High unless switched.
• Documents Telegram /fast as a current-conversation toggle: /fast turns Fast on, /fast again returns to High.
• Documents that Telegram /newchat starts a fresh conversation back in High mode.
• Removes the old account-wide Settings wording and cross-app sync language.

Implementation references

• Backend: https://github.com/premAI-io/premapp-backend/pull/636
• Frontend: https://github.com/premAI-io/fluso-frontend/pull/327

Validation

• git diff --check
• GitHub checks passing: Mintlify build + broken links, cspell

Post-Deploy Monitoring & Validation

• No additional operational monitoring required: documentation-only update.

Latest local verification (2026-06-30)

• Rechecked docs copy after implementation: chat docs describe the High/Fast composer dropdown as chat-specific, mutable between messages, and defaulting new chats to High.
• Rechecked Telegram docs copy: /fast toggles the current Telegram conversation only, /fast again returns to High, and /newchat resets to High.
• No additional docs changes were required in this pass; branch remains clean.

Latest docs clarification (2026-06-30)

• Made the Telegram /fast path explicit: first /fast switches the current Telegram conversation to Fast, running /fast again switches that same conversation back to High, and the bot confirms each change.
• Clarified that /newchat starts a fresh Telegram conversation in High.
• Clarified that app chat mode and Telegram conversation mode are separate: changing one surface does not change the other.